Thank for your replies, fortunately I found a solution. I hope my post will at least be useful for another C3 users (I could have done more research before submitting this post).
Solution was simply to wait couple of hours after publishing my app to Google Play.
https://stackoverflow.com/questions/42783990/admob-real-ads-not-showing-after-publishing-it
Admob plugin was setup correctly - It worked when I published .apk with test mode enabled.