How can I back up using cellular data?

You would need to buy a 5G iPhone. So any model of an iPhone 12, 13 or 14. See -> Use 5G with your iPhone - Apple Support

“Data Mode

• Allow More Data on 5G: Enables higher data-usage features for apps and system tasks. These include higher-quality FaceTime, high-definition content on Apple TV, Apple Music songs and videos, iOS updates over cellular, and automatic iCloud backups.
  • With this setting, your iPhone can automatically use 5G instead of Wi-Fi when Wi-Fi connectivity is slow or insecure on a network you visit occasionally. To turn this feature off and on for a certain network, go to Settings > Wi-Fi. Tap the info button next to the Wi-Fi network and tap Use Wi-Fi for Internet.“

Why would you need to do that? iCloud backups require a Wi-Fi connection, cellular data connection are not possible. Even if possible iOS backups would eat up your cellular data plan in no time flat.

How to back up your iPhone or iPad with iCloud - Apple Support (CA)