Since my router does not seem to allow port forwarding I use a program called radmin vpn to get around this problem. I have used it with many other games and it works perfectly.
When my friend connects nothing appears to happen at the game itself, but I see at the LOVE console "player 1has connected".
I also tried opening a second instance to connect to my own server. Using 127.0.0.1 crashes the game. Using the local network ip puts me in a empty level with 2 players and no walls.