I found this tutorial : https://www.scirra.com/tutorials/1386/u ... lk/page-10
and tested it in a new very simple game project (3 squares activating or disabling ads). It's working! I can see the test ads on my Android tablet.
Now I'll try to make them work in my game project. Not sure how I'll do it as the workflow is exactly the same. I think from what @guannstar is explaining that it should be a problem in my code. "On start of layout" seems to be the cause of his problems so I'll double check that.