Navigate to Slides > All items > edit Slide > scroll down to Slide Settings > Background tab and you can upload background image for each devices for better display https://prnt.sc/-Uwahg0stN71
You are using same background image and settings for both Desktop and Mobile devices https://prnt.sc/5DDR9GJRCL1b
It’s hard to use same background image for all devices. So, I kindly recommend you preparing images you want before uploading them to your media library using the graphic editor (Photoshop or any other) to have a nice-looking grid on mobile devices.