I have just migrated my project that uses GCM to Firebase, by following this guide. At the end of the migration guide it mentions that optionally one can replace the Google server endpoints with the ones from Firebase. However, when I try to send messages between devices, they are not delivered.
The only case that works is when I replace my old GOOGLE_API_KEY with the one I got from Firebase but with Google's endpoints!
From what I see in this link others also face similar issues. Has anyone any hint on this?