What if you don’t have WiFi?

Options are to use a hotspot, public WiFi, friend/families WiFi, or backup to a computer with iTunes/Finder.

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

If your computer doesn’t already have it installed or the current version you’ll need Internet, WiFi or ethernet connection, to install or update it. Otherwise when backing up to a computer you will not need an Internet connection.