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

There is only an option to do that on iPhone models that supports 5G (the 3rd generation SE does) with some carriers. 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.

Also see the information in Use 5G with your iPhone - Apple Support about the settings at Settings > Cellular > Cellular Data Options or Settings > Mobile Data > Mobile Data Options including the following:

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.