settings for XR 15.6.1 for 30" prints of art

Why can't I set the dpi to 300? Printers want at least that, in the photo capture, not an app or photoshop. I can't get 4000dpi as someone said was possible with this phone. What setting give me that? I've tried youtube advice, but large images are still blurred. I need top quality for a printer to do 30"x40". Any suggestions?