I'm in the same boat. I signed up to AdMob only a few days ago, so I imagine I'm getting their new version. I was getting the demo ads appearing, but as soon as I put them on hold, nothing is connecting.
I've linked my AdMob account to MoPub, as well as putting in my ID number where requested... but sadly nothing. The game just runs now as if it wasn't going to use ads in the first place :(