I\'m trying to make a peer to peer Javascript game that can be played on mobile browsers.
There are a few different factors that could at at play here.
I would determine what NAT type you are behind on each side, you can read more about that here https://webrtchacks.com/symmetric-nat. If both networks are behind symmetric NATs you are going to need a TURN server.
If you don't have a browser you could also use Pion TURN a Go TURN client and server.
I would also check when gathering candidates if there is an intersection on IPv4/IPv6. Some phone providers are only giving out IPv6.
UDP may not be allowed at all. This isn't common, but possible. In this case you will be forced to use TURN. NAT traversal via TCP is possible, but not supported by WebRTC AFAIK.