I tried building the syncthing-android apk locally using the instructions given @ https://github.com/syncthing/syncthing-android. All steps worked well and I was able to run syncthing successfully on 2 of my android phones. Both were using mobile data, and hence were not on the same LAN. However, they were not able to connect using the relay server either. Relay works fine when both these phones had google-play-store installed Syncthing.
My questions are the following:
Q1. Is it correct to assume, the community relay cluster supports only official downloads of syncthing executables?
Q2. If yes, could you help me with the details of what is the authorisation mechanism used to ensure such behaviour? This is so that I can emulate the same in my personal relay service.