The only way I got it working is having the ad show all the time. The nice thing is, the ad actually doesn't overlap the game screen, it scales the screen to make room for ad. At least it was nice in my situation.
The only other option is to use CocoonJS with ads, which means you have to setup a mopub account and then add in your admob account and shut off everything else in mopub so you just serve admob ads. It's a pain, but it works. Then you put ads wherever you want.