Google Photos will not backup

I have a Samsung Galaxy Note 8 running Android 9 and One UI. Google Photos will not back up anything. All I am getting as a Getting ready to back up and 185 items left perpetual spinner at the top of the app. When I attempt to manually back up a single photo or a group of photos, I receive the message Will upload when connected to Wi-Fi. I have unlimited data and am perfectly happy with it backing up over my cellular data connection. However, going into Settings->Back up & sync->Cellular Data Back Up, I see that **both** Photos and Videos are enabled. So it **should** be backing up over cellular data. But it's not. What am I missing?