Are you sure that the banners are not visible in test mode? Because when I created a banner 1 week ago with the Admob account I created 2 years ago and selected the TestMode option in the project, the ads were appearing as "Test Ads". I created a new Admob account, everything seems fine but the banners are still not visible. It may be related to the v397.2 version, I don't know.
When I first wrote, I said: "app is live but I can't show any ads/test ads." In other words, ads are not appearing even in the application I exported for release. It also does not work in the other application I created as a test.
So, you can see everything is OK. I got export release and is not appearing again.