Keep an image on iPad for 15 minutes . . . .

I understand that 15 minutes is the limit, but under "Display & Brightness" in Settings (regardless of whether or not it's in the "Lock" or "Unlock" position, I cannot set the "Auto-Lock" -- it says (in faded grey) "2 minutes" -- but I'm getting almost instant fade and can't change that figure to 15 minutes. Desperate here since I need to see photo images for a sustained period of time. iPad OS 15.3 (iPad 7) Thank you.