If your ISP puts you in a CGN (Carrier-Grade NAT), which is going popular, you have no chance of opening public server. You should get in contact with your ISP to fix this. Additionally if you want to check if you are in CGN, go your router interface, if the WAN IP (generally starts with 100. or 10. for CGN) shown here and a IP service like http://whatismyip.com is NOT the same, you are in a CGN.
=== If you are sure you are not in a CGN network, try steps below. ===
Often, installed antivirus may prevent you from hosting your own server. Try disabling it for test purposes.
Make sure your GSA port is opened as UDP. And if you are using "GameType" in "server.ini" as "GameSpy", open "GameSpyQueryPort" in "server.ini" as "UDP" in router interface. If you are using "EnableGameSpy" in "da.ini" as "1", open "GameSpyQueryPort" in "da.ini" as UDP in router interface. DO NOT have both of them GameSpy, as this may result showing your server twice in listing. I recommend you to leave "GameType" in "server.ini" to "LAN", and use the "da.ini" GameSpy feature.
Make sure your game port is opened as UDP.
