By enabling that option your server will be setup to lan i guess, you gotta forward these ports (1716 and 1717 , tcp and udp). First find your router manufactur and router access details ,if you dont know the router login details go http://www.routerpasswords.com/ and select your router. When you have your router access details open a command prompt and type ipconfig , check your default gateway(it is your routers ip).Copy your gateway and paste in your internet browser , enter router login details and look for something like port forwarding