Even on desktop you run into people with weird configs. For example my router will not allow mDNS to work but my ISP NAT is pretty good so I can connect two machines as long as they’re not on the same network! In the end you almost always will need a relay to guarantee connectivity for any random pairing of participants in a large enough population.

The good news is that WebRTC is perfectly straightforward if all you want to do is to connect to a server on the open internet.