Rtte canon.

To start, it is siliva, not gel because movies established Silvia first, it also allows other dragons fire to be used as seen in httyd 2 extended flying with mother deleted scene.

"We've delt with his kind before" isn't a Viggo reference, it's a "Hiccup assumes Grimmel is like Drago" type things, the movies and the shows don't interact.
rtte breaks far more canon than you realise, not even headcanon can make it work.

now let's get into both in universe and out of universe proof it isn't canon.

In rtte, Hiccup and the gan learn a lot of things about the king of dragons as seen here:

Fishlegs: Now, also in Oswald's notes is the King's ability to use mind control over other dragons. This dragon doesn't do that or anything close to it.

Fishlegs: The map, it's changing. [The fire causes the map to before settling and showing an image of the King of Dragons being located under Berserker Island] Of course! The map was leading us to the Titan Wing Dramillion because its fire is the last piece of the puzzle! The real King of Dragons is located [Gasps] Hiccup, we need to get to Berserker Island.

After leaning the name the learns the baby's are immune:

Fishlegs: How can baby Razorwhips fly over the center of the island?

Hiccup: Maybe because their senses haven't fully developed, it helps offset the effects?

Bewilderbeast has a egg, which is given to Valka something she doesn't have in 2:

Hiccup: Toothless! It's okay, bud. Just find another way around. Whoa. Oh, my Thor. The King of Dragons has laid an egg?
(Hiccup drops his sword and shield, and when the future chieftain is about to grab the egg, he was confronted by the unexpected guests.)

Hiccup then finds out about the ice breath:

(Before Hiccup can meet his end, The King of Dragons saves Hiccup and blasts Johann with his icy breath, causing Hiccup and Johann to scream.)

Hiccup: No!

(Once the breath stops, Hiccup looks down and notices ice on his back and when he turns around, the ice cracks. Hiccup looks down at the King of Dragons, who growls silently. Hiccup was surprised, but when he turns around, he sees frozen (and killed) Johann, still lifting the spear and giving a furious/hateful face. Baffled by Johann's apparent death, Hiccup tries to get back up there, but his sword is sliced off by the ice and falls into the hole, but hangs on.)

Rtte also suggests, Drago doesn't have his Bewilderbeast, expect he does. More on this later:

Drago Bludvist: (grumpily and furiously) I don't want excuses! I want the King of Dragons!

Krogan: (scared) I found one, Drago! I did! Hiding under an island in a cavern of ice.

What's the issue? You may ask, well the thing is in httyd 2 Hiccup doesn't know these things:

Hiccup: You think we did this?

Eret: Dragon trapping is hard enough work as it is, without do-gooder dragon riders sneaking in to rescue them.Hiccup: What do-gooder-- there are other dragon riders?
Eret: You mean, other than your thieving friend from last night? You tell me. You may have an ice-spitting dragon on your side, but we still have a quota to fill. How do you suppose we explain this mess to Drago Bludvist?

Hiccup: Drago what-fist? Does anything you say make sense?

Teeny: He's expecting a new shipment of dragons for his army by tomorrow.

No-Name: And Drago don't take well to excuses.

Eret: This is what he gave me last time I showed up empty-handed.[Shows a scar on his shoulder from under his shirt]
Eret: He promised to be far less understanding in the future.

Hiccup has no idea of other riders or ice breath:

Hiccup: Look, we don't know anything about a dragon thief, or an ice-spitting dragon.

Valka is the rider and tells him these things:

Valka: Oh, Cloudjumper never meant to harm me. He must’ve thought I belonged here... In the home of the great Bewilderbeast! The Alpha species. One of very few that still exist. Every nest has its queen, but this is the king of all dragons.

 [Valka shows Hiccup the center of the nest, where an immense, white dragon is asleep in a pool of water. Smaller dragons fly by and respectfully bow before the king.]

Valka: With his icy breath, this graceful giant built our nest. A safe haven for dragons everywhere.

Hiccup: Wait, that’s the ice-spitter? He’s responsible for all that destruction?

She even tells him baby's are immune:

Valka: He protects us. We all live under his care. And his command. All but the babies, of course... who listen to no one!

Drago shows him the alpha controls other dragons:

Drago: NO! Let ME show YOU.

[Drago summons his Alpha again.]

Stoick: What…? Hiccup!

Stoick: Come on, Gobber!

Gobber: Okay, change of plans.

Drago: No dragon can resist the Alpha's command. So he who controls the Alpha, controls them all.

[Drago commands the Alpha to take control of Toothless, who starts screaming and thrashing in pain]

Hiccup: Toothless? You okay, bud? What’s going on?

Drago: Witness true strength. The strength of will over others. In the face of it, you are nothing!

As you can see httyd 2 Hiccup has no idea of things rtte makes him aware of. Already making them incompatible canons. But there is more.

The Flightsuit couldn't exist when it does in rtte because it was a brand new invention at the time of httyd 2. Once again rtte has it way to early and thus more proof it isn't canon.

Bonnie Arnold: Another thing is the story point that, Dean, you were trying to set up is, Hiccup and Toothless trying to reconnect in flight, that will pay off at the end of the movie. They keep trying and failing.

Dean DeBlois: The idea of this whole flightsuit thing is they're working it out. It's a new invention and it ends in a lot of nasty spills but eventually they will get it right.

Now then the next thing, is something not even a headcanon could fix. Rtte literally goes against established canon.

See here:

The entire series is based on the six main Dragon Riders moving away from Berk for over a year. However, in commentary, Dean DeBlois has said that they all lived on Berk between the first two films. The second film’s Art Book elaborates on this saying that they all had responsibilities on Berk which prevented them from leaving.

The series also focuses on the six main Dragon Riders exploring lands outside the Archipelago together, while both Dean and the second film’s Art Book state that only Hiccup and Astrid ventured outside the Archipelago as they were the ones drawing up Hiccup's map. The other four stayed much closer to Berk.

Numerous other "dragon riders" are introduced throughout the series, when the second and third films themselves stress how Berk is the only community who rides dragons. Dean elaborated on this during a convention panel explaining that that’s why Eret and Drago were so alarmed when seeing Vikings riding on dragons, and why Eret assumed Valka was one of them.

What do you say to that? Because those things are straight up truths rtte broke, you can't explain the staying at Berk for five years and also be at the edge it is literally impossible.

Next, rtte fails at Hiccup and Astrid's relationship. They literally make it not just inconsistent with the movies but also worse:

Astrid says she and Hiccup are “just friends”, when a statement made by Dean DeBlois during a panel at San Diego International Comic Con in 2014 confirms that she and Hiccup have been romantically involved since the end of the first film. The second film’s Art Book reinforces this by listing “getting the girl” as one of the achievements Hiccup made in the first film.

Hiccup and Astrid are supposed to have started their romantic relationship in "Blindsided", which takes place only a month or two before the events of the second film pick up. However, Dean DeBlois in commentary has stated that they had been together since the ending of the first film, and it can clearly be seen that they were already romantically involved by the events of Gift of the Night Fury. The second film’s Art Book reinforces the statement that Hiccup "got the girl" in the first film.

Next, I want to show you the show can't even be consistent with itself, what makes you think it canbe with the movies if it can't even keep it's own canon in line? See:

Oswald's letter to Dagur and his drawing on the wall suggest that when he disappeared, Heather was still on Berserker Island with Dagur.
Given Oswald last visited Berk to sign a peace treaty the year before "Twinsanity" took place, Heather would have been about 14 when he disappeared. But in the earlier episode "Have Dragon Will Travel, Part 2", Heather explains how she was so young when she last saw Oswald that she barely remembers him, and she was set "adrift" by Dagur at that very young age and adopted by her adoptive parents.

Back to the Drago Bewilderbeast issue:

Drago is suggested to not have his Bewilderbeast yet, while in multiple sources including the second film’s Art Book, the official website, and many companion books it’s mentioned that he acquired his Bewilderbeast as a hatchling during one of his "earliest conquests".

And the Valka egg issue:

The egg that Valka acquires is suggested to be her Bewilderbeast, who’s not yet hatched. While in multiple other sources including the second film’s Art Book, the official website, and the film’s companion books it’s mentioned that her dragon is older than Drago’s Bewilderbeast, and as stated on the official website and the movie crew's dragon species guide, his is a Titan Wing, making Valka's a Titan Wing as well. In the film itself, Valka mentions how she’s lived with the Bewilderbeast since her abduction 20 years earlier.

So that's all I have to say, as you can see by both out of universe and in universe rtte cannot be canon in any way.

Like I said if rtte can't even keep its own canon straight, there's no way it kept the movie canon in mind.

So no rtte is not canon at all, no matter how you spin it somethings are unavoidable.

That's my reasoning for why, more than you gave for why it is not.

That's all I have for today.

Have a fun day or night.

Your Friend -

Dart_Lover_HTTYD