Setting up MoPub to mediate AdMob native ads

Question

I am trying to serve AdMob ads through the MoPub SDK. So far, I did not succeed, receiving the rather unspecific error message:

No ads found for ad unit.

• I have integrated com.google.android.gms:play-services-ads:18.3.0 and com.mopub.mediation:admob:18.3.0.1 through Gradle
• I included my AdMob application ID in the AndroidManifest.xml
• I created an "Advanced Native Ads" unit in the AdMob dashboard
• I added AdMob as a new network in the MoPub console and inserted the ID from the AdMob "Advanced Native Ads" unit in the field for the MoPub native ad unit
• I deactivated the marketplace to only receive potential AdMob ads
• In my code, I initialize MoPub using .withMediatedNetworkConfiguration(GooglePlayServicesAdapterConfiguration.class.getName(), config)
• In my code, I registered a GooglePlayServicesAdRenderer for my MoPubRecyclerAdapter using a MediaViewBinder

What am I missing here?

In the AdMob mediation guide, it is also suggested to use setLocalExtras when requesting ads. However, this is not possible, if I use the MoPubRecyclerAdapter, is it?

(By the way: Is it normal that if I use the MoPub marketplace for showing native ads, I always get like only one and always the same ad? How to have more variety?)