Android app implement firebase push Messaging

I have good working Android app but notification functionality is stop working before some time (Change from GCM to FCM or vice versa not sure ). Currently we build new version but don't have free developer to handle push notifications fast.

I can provide access to Firebase Console.
Can create what is needed to have a push from server side (just need correct key)

Currently when I try to send push to current version I get a message "error": "MismatchSenderId"