Yeah. There are definitely some problems with notification.
I have 3 «Argus 2» installed.
Usually it just not working not on my phone nor my wife's.
Sometimes it works, usually when the App is open when PIR triggered.
I think sometimes it worked on faded phone when the App was opened not long ago.
It is not critical option for me though and I didn't experiment much with that. But maybe I'll check it later.

OS: Android 8
Location: Ukraine

Count me in with that problem. Bought five Argus 2. And I don't have "Cloud" tab in Reolink App (Android).
I created cloud account via browser and subscribed to Beta plan. But that's it. Maybe it's because i'm not in USA.