Their router only lets them port-forward through a MAC address (this being the "network address" of their ethernet/wi-fi adapter).
Although, yes @thekerbal_alt. Just simply port-forward through your MAC and then you should be good to go. Whether IP forwarding or MAC forwarding, there's no real difference, it pretty much does the same thing.
Otherwise people can just connect to your server using your external IP address, which you can find here.