How to diagnose the error occurs?

[pqnhan]

I got an error message: "It was not possible to establish direct connection with the other peer" when trying to transfer files between two devices on two different networks, although the two devices "see each other".

Is there any way to diagnose the error, find the cause?

[szimek]

Hi! It's best to check for an error in browser's devtools (here are instructions how to find it in Chrome).

However, if it doesn't work then it most likely won't be possible to establish a direct connection :/ The "devices seeing each other" feature is provided by Firebase, so it's unrelated to p2p connection that's used to transfer files.