Unity Ads display placement doesn't work, even with test mode?

I was trying to add display ads (picture) to my Android game after the game over. In the Unity editor it seems to work with the test ad showing, but on my phone nothing worked, neither test or actual ads. If I added a while loop that waits for either the ad being available or 10 seconds passing, it would simply wait for 10 seconds and nothing would show. If I use video ads, on the other hand, it does work – both in test mode and with actual ads.

What is wrong with the display ad placements?

Also, I know you're not supposed to test your app with actual ads enabled, but I only tried that twice or so to see whether anything is actually working with the ads in my app before releasing it.