How to backup icloud using cellular data

Follow this link

How to back up your iPhone, iPad, and iPod touch

https://support.apple.com/en-us/HT203977

You can NOT backup to icloud using cellular data.

WiFi is required to back up to iCloud.

If you have a computer with sufficient free disk space, you can back up to your computer using iTunes.

Also you need a wifi connection other wise back up your device using iTunes on a Mac or PC

You don't.