I can't get push notifications to work on my OnePlus2 (using OxygenOS, a spin-off from Android 5.1). I don't even get in-app notifications for things I should get (the notification pane is completely empty).

The first time I installed the app, I got a textbox about having to install something in order for notifications to function properly. I can't seem to get this message to appear again, despite completely reinstalling the app. I don't recall exactly what it said, as I was a bit too busy to take note of it at the time.