How do I do back up using data and not WiFi ?

Cloud Backup using cellular is only possible in certain cases. See the following from Back up iPhone - Apple Support

Note: On models that support 5G, your carrier may give you the option to back up iPhone using your cellular network. Go to Settings > [your name] > iCloud > iCloud Backup, then turn on or off Backup Over Cellular.

You will have to meet all of the following requirements to update your iPhone over cellular data:

• Have an iPhone 12, iPhone 13 or an iPhone 14
• Have iOS version 15.x or higher installed on the iPhone
• Have 5G service active on the iPhone 
• Have Update over Cellular checked and set to ON.

If you do not meet the above requirements , then you cannot update your iPhone over a cellular connection. You will need to update your iPhone over a Wi-Fi Network or by connecting your iPhone to a computer with an internet connection.

Axel F.