I have a phone with no notification led -- I'm not clear how an app would work if there's no led light. (EDIT// maybe the light is on the back? Have to confess here I've never had a phone with notification light...)
What are the expected and unexpected behaviours with an app?
Actually to solve this hardware problem, we have apps for it.
Search for "redmi 9 activate notification light" gives many apps link.
These apps actually run in background and check for notification from call, sms etc. and then show a colored circle, bar etc. at set place on the screen, even lock screen.
I tried two, and gave all rights required. It does not work when the actual sms, or miss call happens.
So was looking for the app, if anyone uses here for it.