Ah, OK. Thanks for the explanation. I wish there was some compromise where I could tunnel a local port in such a way that didn't require a server.
That pretty much is (or was, not sure how most games currently handle networking) the main frustration of setting up a game server at home for your friends at their homes.
One solution back then was to use Hamachi to create a "virtual lan network". Btw for some use-cases you could even use "ssh -L <local port>:<remote computer>:<remote port> <user>@<remote ip>" with a server, and certainly nowadays you can get a vps at 12$/year (nothing fancy, but good enough for tunnelling ports).