Ad inspector on iOS not showing up

[Zeljko Markovic]

I registered my iOS test device in AdMob UI to test the app using the Ad Inspector. However, when I try to launch the Ad Inspector using the API, I receive the following error message:

Ad Inspector can only be launched if the device is in test mode. To enable test mode, use the following:

Objective-C

GADMobileAds.sharedInstance.requestConfiguration.testDeviceIdentifiers

Swift

GADMobileAds.sharedInstance().requestConfiguration.testDeviceIdentifiers

Error code:  22

Additionally to this, when an iOS device is added as a test device in the AdMob UI, no ads are loading, with message: "cannot parse response", and error code 2

We have tried with numerous iOS devices but the result has been the same. The issue persists throughout a number of AdMob SDK versions.

I have read through a similar discussion here: https://groups.google.com/g/google-admob-ads-sdk/c/5y6REtZ0hVE 
But there was nothing suggesting the problem cause or the solution.