We play a 3 player co op together. The networking is a bit buggy. 2 of the players run on a local network and the 3rd connects via the internet.
After freeing up all firewalls we port forwarded.
The game uses ports 26503 through 26507.
26503 - client port
26504 - server port
26505 - port on server & client to talk to master server
26507 - master server port
Forward each of these individually and include TCP/UDP
Hope this helps.