I'm not sure if it has something to do with Teredo (as that doesn't seem to be working well on my internet), but when Game First 5 runs, games like Forza Horizon 4 fail to connect to multiplayer.
Turning GameFirst 5 OFF solves the issue.
Perhaps not being the newest GameFirst means that updates to it are not made as much or at all, BUT I'm still writing this as a reminder for an otherwise alright software --
aside from perhaps database update but having the ability to play multiplayer is more important as that is why this Asus solution exists as far as I'm aware...
Any ideas?