Hey,
You need to port forward NWN in your router. But you also need to set up your router so it reserves the local IP address for the server computer.
First you should check what local IP the the server computer has in the router's "connected devices" page.
Then, go to the LAN page and look for an option to "reserve IP address" for the server computer. It should be something like 192.168.x.xxx
Once you have identified the IP, you should look for a port forwarding option.
Enter the name of the service, e.g. NWN, and enter the local IP that you have reserved, and lastly enter the port you use. Default port is 5121.
I hope that helps a bit. '> Good luck.
-Snottling