i want to update ios on cellular

You can use Apple's feedback link: Feedback - iPhone - Apple

Do note however, that since iOS updates are extremely large, several gigabytes in size, unless you have unlimited data on your cellular plan, it can be a very expensive proposition to update over cellular. And many people would just eat through their plans unknowingly if that was allowed.

Also, Wi-Fi is ubiquitous enough that it should not be difficult to find a wifi network you can use to update. A coffee shop, a restaurant, your office etc...

You cannot. iOS updates require Wifi or a computer.

The only way around this, is to use personal hotspot from another device.

In this day and age, you certainly have a friend with a computer. If not, Make an appointment at your nearest Apple Store for assistance with the update.