Big system problems that arise during or after a major system upgrade generally need more complex remedies, such as system restoration or downgrade. iTunes plays a huge influence in this. You'll need a Windows or Mac computer with the most recent version of iTunes installed to continue troubleshooting your locked iPhone. When you're ready to restore your iPhone 8 in recovery mode, follow these steps:

1. To begin, turn off your iPhone.
2. Connect your phone to the computer with the Apple USB/Lightning connector while it is switched off. Make sure your computer isn't running any other programs but iTunes.
3. Press and release the Volume Up button immediately after your iPhone is connected.
4. Then tap and hold the Volume Down button for a few seconds.
5. Finally, keep the Side/Power button down until the phone restarts. Even if you see the Apple logo, keep pressing the Side/Power button.
6. When you see the Connect to iTunes screen, release the Side/Power button. Your device has entered recovery mode, as shown by this screen.
7. When you open iTunes, you should get a notification that says, 'There is an issue with the iPhone that necessitates it being updated or restored.'
8. To restore your iPhone from a recent iOS backup, select Restore and then follow the onscreen steps.