If you don't like to use automatic port forwarding or it simply does not work you can manual forward your ports. There is no general solution to do that because every router is a bit different. You will find your solution by just searching "open ports <your router model>" in the web. You will have to open the port you set in the server settings and the next to ports above the port you configured.
If you configured 27015 in the server settings. Then you will need to open 27015, 27016 and 27018 in your router.