How to iCloud Backup using data instead of only using wifi
How do I iCloud backup using my mobile data instead of using Wi-Fi only to backup
iPhone 7, iOS 15
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There is no option to do a backup to iCloud over cellular data with an iPhone 7. With later models that support 5G cellular it is possible with some carriers. 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.