Software update

If you had a macbook you could connect macbook to internet on the phone's hotspot by USB cable. You initiate the update in itunes/finder and once the download finishes the macbook talks to the phone directly thru USB and installs the update. I do it all the time as I have poor wifi and unlimited phone allowance but as you say Apple policy will not allow OTA updates on cellular.

Now, I believe the ipad pro has a lot of mac functions and USBC so you may be able to do that as though it was a Mac.

You need to do updates using a wifi connection the device will not update using Cellular Data

Because you are not using normal Wi-Fi, you will have to update using a computer.

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