iOS app from AdMob error = com.google.DummyAdapter?

In my case I was using Google's Demo ad units. Test ads worked on real devices but on simulators I got the same error as you.

The solution was to use real ad unit IDs and Enabling test devices

AdMob sends back a mediation list with only "com.google.DummyAdapter" when the actual network list returned to the app is empty. This is a special case, and it essentially just means "no fill".

It is quite possible that this happens if you set an eCPM floor and AdMob is your only network. For example, if you set a floor of 1.00 and AdMob does not have an ad worth $1.00 eCPM to show, AdMob excludes itself from your mediation chain.

In this example I'd still expect iAd to be returned. As for why it wasn't, maybe your ad unit had country level settings with iAd disabled in your country (wild guess).