It is definetly something to do with your internet connection or the way the FH login page is connecting to the game.
If this helps or gives you any idea, my internet does sometimes drop on some nights, so I find it quite tricky to stay connected to places myself on some occasions. I'm not quite sure what to suggest, but you might have to reset your router again. If you were able to play FeralHeart perfectly fine the other day, then the only thing that would have change is your internet strength, in which case you might need to ask the person paying for your internet to do something about this and contact your internet provider and they might be able to provide you with an accurate answer or a solution.
Just keep an eye on it I suppose, and make sure you're always logged in on that page and all your details are correct. If you keep seeing the login box on the login page, then you have either entered in a detail incorrectly, or your internet connection cannot handle or connect properly.
I really hope this issue gets sorted for you soon. Internet issues can be a real pain.
Glad the issue is resolved! Continue having fun adventures in-game with the rest of us.