REMEMBER TO BACKUP YOUR PHONE FIRST!
1)Download iMazing (I used version 2.12.3.0 for Windows, Warning: you need the paid version to do this!)
2)Plug your phone
3)Open iMazing
4)Scroll down the second scrollbar and click "Supervision"
5)Click Next
6)Click the Organization drop down menu and select "Choose..."
7)Click the + button
8)Write something on the "Name" field, leave other options as defaults and click "Save", select the organization just created and then click "Choose".
9)Leave all options as Default and click Next.
10)Click Next
11)Digit the number above in the field to confirm the action.
THIS STEP WILL ERASE ALL YOUR DATA, DO IT ONLY IF YOU HAVE A BACKUP!
12)Click Next
13)If the program tells to disable Find my iPhone, disable it.
14)On the phone, do the initial configuration setup (the only thing necessary is Wi-Fi).
15)Download this (https://cdn.discordapp.com/attachments/688122358107603013/829323445200355359/90_Day_Delay.mobileconfig) on safari
16)Make sure you have enabled OTA updates (Certainly they are enabled because of the reset)
17)Go to Settings -> General -> Profiles (under VPN) -> Untitled -> Install (top right)
18)Go to Settings -> General -> Software Update. There we go! Install iOS 14.3

Feel free to modify or public everywhere! Even without credits