Server not responding for friends

I have recently got a secondhand computer and tried to turn it into a GMod dedicated server. But as the title says, the server is not responding. The problem probably lies in the ip
Im behind a router and my external IP is diffrent than my internal. The server uses his internal IP, and people can’t connect to that. The ports should be forwarded correctly, but it’s not working. Can anyone help me?

Tell your friends your external, not internal. and add 27015 or whatever your port is