Curious which particular credentials are you using? I only ask because within MoPub, you should be using your AdMob's 'Ad Unit ID' and not the 'Legacy Publisher ID.' I know it may be confusing as within MoPub's dashboard it mentions to have the 'Publisher ID' but really it should be the 'Ad Unit ID' from AdMob that should be entered.
Please consult this guide for more info: http://help.mopub.com/customer/portal/articles/1376821-how-to-set-up-updated-admob-revenue-reporting