If you can see/play with your setup on the LAN, then you can try our method for Internet as well.
We have our Internet providers, well, provide us with 'white' IP, however temporary (so it doesn't come from 192.168... etc ranges). You can check it with any "what is my IP" service. This IP should be the same as the one you got on WAN interface of your router (go to settings page of you router and see what is in WAN).
I believe this is a common practice nowadays for ISPs, so you could try the following right away:
a) Set up a port-forwarding on your router for port 7808 to your server inner (LAN) IP for both TCP/UDP and call it to say "Freespace";
b) let you friend in the Internet go to "f2" "multi" options in F2 itself and add there your external IP (as I said it could be known with any web service for it), don't forget to hit "Enter" after you have typed it. Make sure that it is stayed in the list (go to those settings again). Important: you should restart the game after that.
This way your friend should be able to see your server in the list of available ones.