Yes you must create it every time you enter the layout. I don't know how you managed to show ads on remote preview, seems impossible.
I dont think this is a good idea... it might generate too many ad requests and decrease match rate... also this might cause the game to lag, because loading an ad will consume network and CPU...
I have had success creating each unity just ONCE and using the SHOW action when I want to show them, it still seems to be working like this EXCEPT for banner ads, now banner seem to work just once per creation..
I think Construct needs to update its manual, and actually take a good look into using AdMob... Cause I dont think the team really finds it relevant for them or I dont know, there's always some confusion... for example if an ad can be shown only once and cannot be called again before creating the ad unity again, then instead of using HIDE AD it should say DESTROY AD to make more sense in Construct logic. Because HIDE/SHOW makes you think you can do as simple as that, but no, if HIDE means the ad will be destroyed and created again, than the action should be named DESTROY AD...
At least for the Banners now, it seem to me that Hiding the Banner will really DESTROY it and they wont come back. But I can show Intersetial more than once from creating the ad unity just once at the loading screen...
The manual for the ad plugin is too vague, I find many holes in it... Can be really frustrating testing over and over to make sure you get what you want
I will mention the boss Ashley here just in hope to get some clarification or useful comment...