Want to download ios15
I want to download iOS 15 using my cellular data
iPhone XS Max, iOS 14
It is not possible to download iOS 15 using cellular data directly.
You could try to connect the iPhone to your laptop, download the update into your laptop using your iPhone's hotspot and then install it from there (through iTunes or Finder).
Not certain if this will work or not. Also, the iOS 15 update file is around 6GB+, so I would only try this if you have an unlimited data plan.