Steps:
1. Run cmd.
2. Type ipconfig, find this:
3. On the cmd, also find your default gateway (which you have already done)
4. Make sure your server has the same port by doing
echo($Server::Port);, also make sure UPnP is disabled.
5. When port forwarding, make sure you only do the same range with UDP. (Ex: I have 2 servers)
(Optional) 6. If you want to have a better chance of people connecting, try port triggering. People can send challenge requests and even ping my server after I port triggered. I don't think you need to make it UDP only, since "both" works fine for me.