Do you have to have a sentinel with you when running COIN or does COIN also act like a sentinel? Can COIN run in the background on your phone? It seems that I don’t get rewards unless it’s the app on top.
No, you need to keep it up and open to COIN right now (verified by Johnny over at XYO’s Reddit page). If you are on an Android, you can enable developer mode and select the option to never go to sleep while charging (if you don’t want to try to find an app). I basically leave my phone charging (battery is shot any way on my old phone) when stationary. When on the move, I typically plug in my portable charger.