Do you use microg?
Signal can work with GCM (by default) or use websocket. It depends on if the app sees GCM on your phone when you open it for the first time.
If you use microg (and everything is configured as it should), then Signal works with GCM.
I would suggest to change the mode from GCM to Websocket (or opposite) to test if there is a difference. To do so, you need to reinstall Signal. I have read (but not tried myself) that you can change the advanced settings in Signal after having changed the GCM option with microg: switch off and on again “Signal messages and calls”.
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