Sending push notifications when the player has been inactive for a while

Hello everyone!

I'm looking for a way to send notifications on a mobile phone when someone hasn't played the game for some time.

I've been looking into mobile notifications from a Unity game but haven't found anything I can really use. I have found (non-free) assets that give you the ability to do this in the store but I'm looking for a way to do this myself (in code).

If possible, the solution should work for IOS as well.

All help is appreciated

Thanks in advance!