And yet another hint: I had to dial the sequence documented in https://github.com/microg/android_packages_apps_GmsCore/issues/740#issuecomment-479252876 in order to get GCM working correctly. Before that, apps had not been able to register with GCM.
And yet another hint: I had to dial the sequence documented in https://github.com/microg/android_packages_apps_GmsCore/issues/740#issuecomment-479252876 in order to get GCM working correctly. Before that, apps had not been able to register with GCM.