Can I back up using data
Can I use my data to back up to iCloud
iPhone 6s, iOS 13
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No. You must be connected to WiFi in order to backup to iCloud.
How to back up your iPhone, iPad, and iPod touch - Apple Support
You can test this by turning off WiFi and try to start an iCloud backup, you will see the message in iCloud settings that you must be connected to WiFi in order to backup.