How do I back up my iPhone using my mobile data instead of my Wi-Fi?

That is not currently an option.

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

You can't do it wirelessly (to iCloud). A reliable and fast wifi network connection with internet access is mandatory (no cellular data, no personal hotspot, no VPN connection).

You can do it via computer running iTunes (PC) or iTunes/Finder (Mac), bearing in mind that the computer must be connected to an wifi network with internet access.

Regards

Giulio