Have you forwarded the ports in your router too?
So it does work at least in your internal network...
Are you referring to the in game server list right?
No, this won't work...
Do you have other services which are running on your server that are reachable from your server?
Yes and no...
You first have to make sure that you can forward the ports and your ISP is not using a CG-NAT.
Then you have to forward the ports and you can then give your friends your public IP to connect, so to speak if you forward the ports it registers at the master server and the master server checks back if the server is reachable from the outside world, if not it would not be populated in the main in game server list if it is reachable from the outside world you should see it in the main in game server list.
I would recommend that you first look into Port forwarding itself @SpaceInvaderOne has also some good videos about it, but please note that these videos are mostly about SWAG (former LetsEncrypt) but it should be very much similar.
The important part is to look into how you forward ports on your router.