For clients:
Get https://ufile.io/lnai5
Go to Risk of Rain 2\Risk of Rain 2_Data\Managed
Backup assembly-csharp.dll
Replace it with the downloaded one (you'll have to rename new)

For hosts:
Do the above
Either make an autoexec.cfg file in Risk of Rain 2\Risk of Rain 2_Data\Config that has "sv_maxplayers 16;" (no " obviously) or write it in your console every time you boot the game (ctrl+alt+~ to open console), you can check if it registered by entering sv_maxplayers in your console, it should say back "sv_maxplayers" = "16"
When you host, immediately go to private lobby (character selection screen), don't wait in the screen where it gives you the options. After that, post your lobby ID in the thread