I am stuck in diagnostic mode after iOS upgrade on my iPhone 12

Hi cuhullainn,

If the iPhone will not fully power up or progress past asking for a Wi-Fi network, you'd either need to connect to one or if you have access to a computer you may be able to try restoring the iPhone via iTunes/Finder.

See more info here: If your iPhone won't turn on or is frozen - Apple Support

"If your iPhone turns on but gets stuck during start up

If you see the Apple logo or a red or blue screen during startup, try these steps:

1. Connect your iPhone to a computer. On a Mac with macOS Catalina, open the Finder. On a Mac with macOS Mojave or earlier, or on a PC, open iTunes. Find out which macOS your Mac is using.
2. Select your iPhone on your computer.
3. While your iPhone is connected, follow these steps:

• On your iPhone 8 or later, including iPhone SE (2nd and 3rd generation): Press and quickly release the volume up button. Press and quickly release the volume down button. Then press and hold the side button.
• 
  
  • 
• On your iPhone 7 or iPhone 7 Plus: Press and hold the side button and volume down button at the same time.
  • 
  • On your iPhone 6s or earlier, including iPhone SE (1st generation): Press and hold both the Home button and the side button or the top button at the same time.

1. Keep holding the buttons until you see the recovery mode screen.
2. 
3. When you get the option to restore or update, choose Update.
4. 
5. Your computer will download the software for your device. If it takes more than 15 minutes, your device will exit recovery mode. Repeat steps 2-4."

This process attempts to reinstall iOS without wiping the data on the device.

Let us know if that helps.

Cheers!