Update over cellular instead of wifi

As you already know that iOS can only be updated over a Wi-Fi Network and not over the cellular network. There is other way that you can update your iPhone and that is with the use of a computer. The computer must be connected to the Internet however it can use a cellular hotspot if necessary and does not have to use Wi-Fi if Wi-Fi is not available.

Here is ➔ How to update your iPhone with a computer (Mac or Windows with iTunes). Scroll down to the section "Update your Device with a computer" section in the link.

Axel F.