teux check if it is running in the background by swiping down from the top of your phone twice to see all the quick settings tiles, then at the bottom you'll likely find a circular icon with a number in it. Tap that and if you see Signal there, try stopping it.
If you have sandboxed Google Play installed, you might consider uninstalling Signal, then install it with Google Play so it can get notifications via FCM.
Make sure you back up your data before you do that, though.