iOS updates are huge. Ensure you have a high-capacity or unlimited data plan on the hotspot device to avoid overage charges or the update failing halfway through. Troubleshooting: iPhone Still Demands Wi-Fi
Go to Settings > Cellular > Data Options, turn off "Allow More Data on 5G" and then turn it back on. how to download software update without wifi iphone
If you have another mobile device, you can use its data to create a Wi-Fi network that your iPhone will treat as home Wi-Fi. iOS updates are huge