Many apps, like Signal, have developed alternative solutions to things like push notifications when Google APIs are not found.
Discussion
There's also https://unifiedpush.org/ so you don't have to roll your own push notification library.
This is the way.
unified push is dev-friendly, cross-platform, awesome non-FCM push notifications. Sample android app integration on their github here: https://github.com/UnifiedPush/android-example
test (sorry!)