Can I back up over cellular data
Can I back up my iCloud over cellular data
iPhone 11 Pro Max, iOS 15
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You can backup over cellular if you have an iPhone 12 or 13 connected to 5G with a carrier that supports the option.
Link-> 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.
The option won't be there if you are not using an iPhone 12 or 13 on a carrier that supports it.