Why does my iPhone XR keeps asking for WiFi when I have an unlimited data plan

Large transfers such as iOS updates and installing large apps require Wi-Fi for several reasons:

• The most important one is that carriers require it to prevent overload to their networks. This is not an Apple decision, it is a condition for using the cellular network. Cellular networks run close to capacity most of the time. When a new update becomes available hundreds of millions of users all want it almost immediately. So throwing those large volumes of additional data demands on the network can crash it. Each tower can support a few dozen connections simultaneously. When hundreds of your neighbors want to update around the same time no one will get an update.
• From your perspective, cellular data transfers are slow. And “unlimited” plans are only unlimited in terms of total volume of data, not speed. Most “unlimited” plans have an upper limit on download speed, typically 3 Mbps. Fast enough for email, games and web browsing, but a large download may take hours, and if it is interrupted it needs to start over. That’s why I don’t have an unlimited plan; my 12 GB plan has not speed limits, and I have seen over 50 Mbps with a strong signal.
• Latency. Try running speedtest.net on your phone and note what it lists for ping. This is the time needed to start transferring a single packet. On my Wi-Fi that is currently 11 ms. Over the cellular network it is usually over 100 ms, which can be longer than the time to transfer a packet. That also makes transferring thousands of packets take much longer.

So the bottom line is that updates require using Wi-Fi for good engineering reasons.

Hello WanderingFirbolg,

Thanks for posting. I understand you have some questions about using cellular data on your iPhone.

To clarify, it's normal to need a Wi-Fi connection for the functions you mentioned. Wi-Fi is necessary to update wirelessly or to download Enhanced Quality voices for Siri. If you're having issues with using cellular data in other apps or situations, like with Safari, you might want to review your cellular data settings. You can do this by tapping Settings > Cellular or Settings > Mobile, depending on your region: View or change cellular settings on iPhone

I hope I've answered your question.

Take care.