I feel sorry for the dev of this fun little game. Literally every comment is about multiplayer not working even tho it works like a charm. If you want to run a game server then obviously you have to open the Required ports!
If you're on windows you dont even have to disable the firewall completely, just allow the game to communicate through it.
There are literally hundreds of Videos on how to do it.
Cheers to the Dev for the Game. We had a lot of fun.