Start a listening service, portforward to it if you're behind a NAT router, then use one of the online port scanners or some other host you have access to from somewhere else on the Internet to attempt connecting to it.

Yes, mobile internet has AFAIK always been behind a NAT --- I'm not sure if you can even get such a service with a publicly routable IP, since there's not much interest in e.g. hosting a server on your phone while you carry it everywhere, as useful as that could be.