How to back up your iPhone without wifi

I think this is only possible if your provider has made it possible.

In this support article it is also explained this way. So it depends on your phone provider.

Back up iPhone – Apple Support (UK)

Note: On models that support 5G, your carrier may give you the option to back up iPhone using your cellular network.

On my iPhone, I have the ability to backup via 5G without wifi.

Unless you have a 5G iPhone, and are connected to a 5G network that supports it, you cannot backup over cellular data. iPhones, other than the 5G models (that is, an iPhone 12 or newer), require a WiFi connection to backup over the air. Even a 5G device on a 4G LTE connection cannot backup over cellular.

You can use a lightening to usb cable plugged into your MS Windows or MacOS computer and use iTunes or finder to backup instead if backing up wirelessly.