In the properties of your game you need write the Id of your App.
Have you see the wiki page ?
http://wiki.compilgames.net/doku.php/gdevelop5/all-features/admob
You will have to wait a few minutes or a few days for google to verify your account, after that Google will propose ads, during this check time you will have nothing.
You may only have banner ads at first. If you have them you will have the reward video and Interstitial don’t worries.
Think just to use the right Id for each type of ads, and your app Id
Each Id is different.
And turn Off test mode before export in APK.
If you have any doubts about your events look at the Admob example provided with GDevelop, these events are functional.
https://editor.gdevelop-app.com/?project=example://admob
I’ve tested all types of ads: Banner, Interstitial, Rewarded Video, all work perfectly.