Hello! :)
I see you are facing problems with multiplayer, let's fix that!
Suggestions:
1- Make sure the host window is not minimized, the host is the server, you minimize the host, the server shutdown, so when you test the multiplayer, make sure both the host and the players windows are open side by side (just like in my YouTube Impostor Alert Template video).
Note: you can also test the game with your friends.
2- Host a public lobby, and then check your firebase database, did a new ID showed up ?
And when the other player open the game, does he see the public lobby in the list ?
3- In MainMenu scene, at the very event at the top, remove the actions (use ICE server) and (Disaple IP sharing), then try the game, we did this to make sure if the problem is caused from metered or not.