Can I not use Cellular Data to back up ?
Can I not use CELLULAR DATA to Back up instead of WiFi?
iPhone XR, iOS 14
Sorry, not on an iPhone XR. See the following from Back up iPhone - Apple Support
Note: On models that support 5G, your carrier may give you the option to back up iPhone using your cellular network. Go to Settings > [your name] > iCloud > iCloud Backup, then turn on or off Backup Over Cellular.