Software update using mobile data

If you want to update the iOS, you MUST be connected to WI-FI. (You can also update using a computer which also requires WI-FI or ethernet.)

Update your iPhone, iPad, or iPod touch - Apple Support

Updating with a mobile data connection is not possible when using iOS version 13. IOS version 15 allows it with a 5G capable iPhone. IPhone 12 and 13.

Axel F.

You cannot do that.

Well, actually you can with newer iPhone models using 5G.

Use 5G with your iPhone - Apple Support