I have an android published app working and serving ads with the app-ads.txt setup correctly. I tried to upgrade the ads library to the latest 'com.google.android.gms:play-services-ads:20.5.0' and now I am getting the error code 3 'No add config' every time I try to load an ad.
Long story short, after much investigation I downloaded the Google ads sample which works okay. If I change the applicationId on the project build.gradle to anything ads loads. If I change it to the id of my app I get the error.
Why the new ad library refuse to serve my app while the published app works okay?
I found the issue not clear on the docs. When you use the admob mediation you need to define the test devices. Mediation will not work with the new test ids.