Problem is simple.
When I install my app to any device direcly with an APK, my admob ads work fine.(real ads)
When I install my app to any device from Google Play Store, ads are not showing.
I think Google Play Store's signing system causing this problem but I can't disable Play Store signing after publish.
Its definitely Google Play Store's signing system. So far I found only one solution. Unpublish the app and then use different package name and opt out play store's signing system, just use your keystore. Hope this helps.