Why you don't use just 1 pc?

You open, for example, port 2300 (protocol: udp) and forward it to your computers lan-ip (make sure you've got static ip address in your network!)

After that, start the server with: -sv -port 2300

If you've started, now start the client with -cl -port 2300 -ip internet-ip

I also recommend you to get a dyndns.

Regards
TSGames