How can I back up my phone if I don’t have Wi-Fi?

I back-up my phone to my Mac using a cable... see this link...

How to back up your iPhone, iPad, and iPod touch - Apple Support

You don't, sorry.

"Borrow" a reliable and fast Wifi network with Internet access from your relatives/friends/neighbors, or ask for Help at an Apple Store or authorized servive provider / reseller.

Regards

Giulio

Do you have a Mac or PC?

Don’t have WiFi how do I back up