Android app not registering with FCM, but Server initialization apparently working

I followed the installation steps in [`README.md`][3], which worked without problems and the project built and ran fine.

I followed the steps in [`ANDROID.md`][4] and added FCM to my project on https://console.firebase.google.com/ (written down `Sender ID` and `Server key`). Then I created an app on https://play.google.com/apps/publish and linked the `Sender ID` in the `Services & APIs` section.
At this moment I wasn't exactly sure what else to do to make Google aware that my app is authorized to register for FCM. I tried uploading my apk in the `internal test track` channel, so it could see the config and signature in the apk, is that how it works?

Then I added the appropriate `Push.configure` and `App.configurePlugin` calls on the client as well as the `Push.configure` call on the Server.
To help debugging and fix potential problems, from [here][5] under the Server Code section, I added `Push.debug = true`, `Push.allow(...)` and a `Meteor.methods(...)` for debug output and triggering a notification sent from the server via the Browser console in the client (which according to Server logs is triggered).

On the Server logs, the Push registration seems to have completed (`GCM configured` log output), but on the client, even with `Push.debug = true`, I get no output on whether anything failed or succeeded.
All I know is that the server doesn't get a device which registers and says `Push: Sent message "the title" to 0 ios apps 0 android apps`, when sending a message through the `Meteor.call`.

Did I forget a step for properly registering my app with FCM and how can I further debug my problem?

[3]: https://github.com/raix/push/blob/master/README.md
[4]: https://github.com/raix/push/blob/master/docs/ANDROID.md
[5]: http://%20https://medium.com/@acarabott/meteor-native-ios-push-notifications-heroku-raix-push-cordova-213f486c4e6d

Start with README.md and docs/ANDROID.md, then review the client Push.configure and App.configurePlugin calls alongside the server Push.configure setup described in the report. Reproduce the missing registration with Push.debug enabled and inspect the server's registration and device output; done means an Android device appears and notifications can be sent to it.

Written by the indexing model from the issue text.