Oh it's a kids game. Does your game comply with all 5 million family friendly requirements?
The highlights: support.google.com/admob/answer/6223431
Google is a pain when it comes to ads in kids games, that could be why it works fine in debug mode and not when downloaded from Google Play.
This is the way I test my games (over explained). I create an apk (NOT debug) and download that to my hard drive, then I upload it to my Gdrive so I can test it on several devices. Once on the Gdrive, I download it to every phone in the house - even the old ones.
I install it on each phone and test it, I especially look for when the ads pop up. Do NOT touch your own ads and do not use them to get stuff, but you can view your ads without pissing off the Google God.
If the apk (not debug) downloaded to your phone gives you real ads when played, but the bundle you uploaded to Google Play doesn't - Google is probably stopping it.
I played about 12 games in your app, but didn't get any ads. I went back to the menu screen several times.
Try the test I mentioned above to see if you get the ads. If you do, Google Play is probably stopping them for some reason.