You don’t need to keep changing your router’s configuration once you’ve set up port forwarding. You should set it up again only if your local IP on machine with nxserver changes. That’s why is better to reserve IP on router for that machine (static).
Sorry, maybe your article was a bit confusing (and my knowledge for sure is limited) because you mention a “public IP” number, and I then took for granted it was the external IP (otherwise why would it be public?). Or am I wrong?
Enabling UPnP makes it easier. If you don’t want to use that, you will have to configure the port forwarding by using the router admin interface. This is normally a Web application running at http://192.168.1.1. Follow these instructions for your parents’ router:
1. Reserve local IP of your parents’ computer on their router according to your router’s instructions.
2. Open a unique port for that local IP in Port Forwarding section on the router.
3. Check public IP of the router.
4. Now in your Player connection settings – enter the public IP of your parents router and unique port. Now you should connect to your parents machine.