After a few hours i found out how. As i thought, you have to open a port under either:
1. External port: 40444 Internal port: 40444 Protocol: UDP
2. External port: 40445 Internal port: 40445 Protocol: TCP
3. External port: 40447 Internal port: 40447 Protocol: TCP/UDP
And finally, you go get ur ip on whatsmyip or something.
Don't thank me, thank this website httpTWODOTS//findportsDOTcom/document.php?tag=nemesis-of-the-roman-empire
I knew there was a few other games that looked like celtic kings from the same devs and thought maybe the ports would be the same and i was right, have fun with your pals.