Hello, From my experience when you create your banner and place the code onto what ever page you want it to display on, it will display on both mobile and desktop automatically unless you state otherwise or If you have 2 separate sites, 1 for mobile and 1 for desktop desktop, if that's the case you just place the code on the 2 corresponding pages.
Or you can add 2 banners and specify one for desktop only and others for mobile only