Since you were able to display ads on a simulator, it's enough confirmation that your code implementation of the Mobile Ads SDK is working correctly. It's possible that there are just no available live ads, at the moment, for your live Ad Unit Id which will throw a
No fill error. You can test our
sample Ad Unit Id on your real device and if you are able to display
test ads, then you can safely assume that there is no issue with your device. Also, enabling the Limit Ad Tracking (Go to
Settings >Privacy >Advertising) in your device could also affect Ad serving on that device.
If you want, we can test your Ad Unit Id on our end if you can provide it to us privately using Reply privately to author option.