You can only back up with WiFi
is it only WiFi that can be used to back up
iPhone 6s Plus, iOS 14
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You can back-up via iTunes.
iCloud Backups require Wifi, you'll need a Wifi connection to use this method (especially if you don't have iTunes installed), you could go to an Apple Store and use their wifi if one is open near you.
How to back up your iPhone, iPad, and iPod touch - Apple Support