AdMob test ads not showing on Android (HTTP 403 error on load)

I am integrating AdMob into my Android application, but ads are not loading at all, even when using Google’s test ad units.

I am receiving the following error in Logcat:

onAdFailedToLoad: Received error HTTP response code: 403

App Details:

Package name: com.remotep.insigniarokuremote

Testing on: Real device

Ad formats: App Open, Interstitial, Banner, Native, Rewarded

What I have tried:

Initialized Mobile Ads SDK properly: MobileAds.initialize(this);

Verified internet connection

Tested with Google test ad unit (e.g. rewarded: ca-app-pub-3940256099942544/5224354917)

Also tested with production ad unit IDs (fetched from server API)

Waited for ad unit activation

Tried different ad formats

Additional Info:

Ad unit IDs are fetched dynamically from backend API

Currently mixing test and production ad unit IDs

App Open ad is loaded during splash screen

What could cause HTTP 403 errors in AdMob ad requests, even when using test ad units?
Is this related to mixing test and real ad units, missing test device configuration, or account/app setup?

Any guidance would be appreciated.