How can I backup using cellular data

You need an iphone capable of 5G (iphone 12 or 13) and backup manually to iCloud in Settings, your name, iCloud, iCloud Backup. Can do it on 4G but the phone needs to be 5G capable. Auto backup does not work.

OR

Tether your computer to your phone's hotspot by USB and backup in iTunes/Finder.

You need an iPhone 12 or 13 on a 5G network to backup using cellular data.

I should add that my assertion that you can use 4G on a 5G capable iphone is based on observation of my own iphone 12 on UK based O2 4G network and it may not work universally in every country.