Hello.
If rewarded ads are running in test mode on PC, then technically the calls to the plugin are being made and the ad can be shown.
However, on a smartphone, the real AdMob plugin is active and it won't let you do silly things. Even in test mode, the plugin won't let you spam ads or ad requests.
When you remove the test mode, the app needs to be reviewed in the AdMob cabinet. Even then you won't be able to spam ads; AdMob will show you a real ad once every 60 seconds, and if the app is new, it will only show a few times per session for that player, and then it won't show any more.
I understand that you see a lot of games on the Play Market that just bombard you with ads, but that's because mediation is working and many ad networks are connected.
So, rewarded ads of this type cannot be used. You can't run them right after startup, and you can't run them every two seconds either. Rewarded ads should be launched after a user action and should provide a reward.
You can simply look higher up in the thread, we provided code there on how to load and show an ad.