What if you don’t have WiFi?

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.

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