Hi @TrZd,
You can add a screen and add a component like image to show the Ad and then you can make that screen as the Home screen and add a countdown that links to the real home screen. And then everytime the user close the all and opens the app again he/she should see that screen and then go to the real home screen.
If this not fixed like you need to show random ads from a collection then you can generate a number and show a list of ads filtered with that number. You need a id for your ads in the ads collection too.
Check this thread : Showing a screen at startup only
Hope this helps!
Thank you