I had the same problem with signal and no GCM-login, and i solved it by entering the following phone number
*#*#2432546#*#*
(found at https://github.com/microg/android_packages_apps_GmsCore/issues/740#issuecomment-478675965 )
my workflow was:
- entering the phone number
- backup signal data (check signal menu)
- Signal App --> stop and delete all data and cache
- stop blokada
- reboot
- wifi off
- entering the phone number again
- start signal and restore from backup
Then Signal and other apps were registered in GCM