Multiplayer Connection Problems: I can't host, but another computer in my home can

So basically when playing Stardew Valley Multiplayer if I host a game eventually anyone connect to me gets booted out. Saying there was a connection error. If I then join a game to a friend say across town I will eventually get booted.

Though if my fiance, who's computer is right next to mine, hosts everyone can join hers without any issues myself included. And while we play no one gets connection issues.

We are both wired and on the same network so I am at a loss. Happy we found a solution to play, but I would like to fix why it wasn't working from my PC.

 

We have a general guide for troubleshooting connection issues (it applies to both vanilla and modded players) found here. The router issues, etc would most likely not apply to you, but go through as much of the guide as you can. Also watch out for antivirus programs and things like Afterburner, which can interfere with the game running.