I've never used the ads in mobile games for two main reasons I'm not trying to discourage you from using them in any way but I think you should know this before using them. Number 1 no major company who publishes games nowadays uses them clash of clans for example. Number 2 as for why not a lot of players are not willing to play games with ads because they're annoying and the reality for you is ads don't pay you anything unless you have millions of players even then pennies per click emphasize the click part. Your much better off adding in-game currency players can purchase you'll make a lot more money$$$.
So first I recommend reading this official documentation construct.net/en/make-games/manuals/construct-3/plugin-reference/mobile-advert.
So you can understand how to trigger the events correctly now its impossible to tell how your game is set up so ill give you a rough idea as best as I can
Players health <or= 0 ---------------Create Banner
system trigger once while true-------
Is Banner Loaded---------------------Show Banner
On Banner Shown----------------------system wait X Seconds
....X being how many seconds you want the player to have to stare at ad now if you use video instead I'm not sure how that works.