Fix problems with push notifications in forked fluffychat App

Hi,

We have forked the Fluffychat app and changed the application id.
But we are now having problems with the background push notifications not arriving on Android devices.
In the original app, the push notifications work without any problems, so it is probably a configuration error on our part
The push setup is Matrix Server -> Sygnal Push Server -> Firebase -> Android App

We are therefore looking for someone very experienced who can solve this problem.
Preferably someone who has already forked the fluffychat app and set up push notifications on their own push server.
In addition, the problem should be solved as quickly as possible, if possible today.

We have already set up a push server which can be used for testing.

This is our forked repo
https://github.com/luckycloud-GmbH/fluffychat/tree/redesign
And this is the original one
https://github.com/krille-chan/fluffychat