I've been testing with iOS and I have managed to get ads showing with MobileAdvert plugin.
Note that I am only testing, I havent tried ads with a published app yet.
I found that the MobileAdvert > Test Mode property doesnt seem to work for iOS in the same way it does for Android.
In order to test ads on iOS, you have two options:
You can use the test ad-unit IDs that AdMob provides (ths is the easiest way to test)
You replace your real ad-unit ID with test ad-unit IDs.
The second option is to add you test device (iPhone, or iPad) to 'Test Devices' in your AdMob account. To do that you need to use an app to retrieve your IDFA number from Apple device. Doing it this way, you can use your real ad-unit IDs.
In both cases, you export your xcode project from C3 > open Xcode on Mac > connect your test device to Mac > build project, game launches on device and you should get ads.