Why can’t I update my phone without WiFi?

There are several reasons for this. I will list a few:

1. Cellular Service providers do not want large downloads on their networks during update times.
2. Large downloads over cellular networks, with multiple users, can slow down speeds by congesting the cell tower.
3. Most people have metered data plans and an accidental update large download can put a dent in their plan.
4. Etc. Etc.

NOTE: There has been a change with the advent of 5G and its spread in the US. As you know, 5G service provides a wider bandwidth and higher speeds allowing more capacity within the spectrum. You can use an iPhone 12/13 with 5G service, AND iOS version 15, to download updates and iCloud backups over a cellular network.

Axel F.

Redplasmab12 Said:

"Why can’t I update my phone without WiFi? Why can’t I update using my cellular data? sounds like a way to make me buy wifi"

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Troubleshooting iPhone Updating:

A. Updating Over Cellular:

You can only update over Cellular using an iPhone 12 or later with 5G. So, updating over Cellular is not be an option for an iPhone XR, because it does not have the hardware required to do so..

See Here: Use an iPhone 12 or later, that is using 5G, in order to update over Cellular - User Tip

B. To Check for Updates:

Use a Computer and Ethernet:

Start with attempting an update through use of a computer. If you cannot use Wi-Fi, then use Ethernet.  Go Here: Update your iPhone Using a Computer - User Tip. On the computer, you would connect it through Ethernet, going directly into your router. For a Mac (or PC) that does not have Ethernet, this would be the adapter to use: Belkin USB-C to Gigabit Ethernet Adapter - Apple. If you get the same error, the model is likely just not compatible.

You are welcome.