Ok, now I'm confused.
I've created a new testapp, only with your example code from "FirebaseAdMob - Admob ads integrated with Firebase backend" and the manifest snippets, created a new project in firebase, created 2 ads in admob, banner and interstitial and what happened? BannerAd was shown instantly, Interstital a few minutes later...
Ok... this shows me it works.
Then I took this project, changed the package name to my appname, changed the google-services.json file and the Ad IDs ... and the Ads were not shown, Errorcode 0... I don't understand what is wrong here... I want to cry
Has anybody an idea? :/
*Edit* I've changed the AD IDs of the testapp to the IDs not working ads app... the ads started to show immediatly... So I think there is some code in my not working ads app that disturbs the receiving of the ads... or (what I don't really believe) the packagename was blocked by admob (is this even possible and would the errorcode still be 0?)