What follows is an unofficial fix for the cellular and Touch ID issues introduced by the iOS 8.0.1 update.
I present this as is, and offer no endorsements or warranties. If it goes wrong, then you're on your own. However, this worked for two devices we tested, the iPhone 6 in the New York newsroom and the iPhone 6 Plus in the UK.
UPDATE: Apple now endorses this method for rolling back to iOS 8.0.
- iOS 8.0 (iPhone 6 Plus): iPhone7,1_8.0_12A366_Restore.ipsw
- iOS 8.0 (iPhone 6): iPhone7,2_8.0_12A365_Restore.ipsw
2. While the file you need is coming in, go into Settings > iCloud and turn off Find My iPhone.
3. Next, connect the iPhone to a PC or Mac running iTunes using a cable connection (not Wi-Fi), and allow the two to connect.
4. Make a local backup using iTunes before you do anything else by clicking on the Backup Now button. Make sure you update to "this computer," rather than iCloud, encrypt the backup if you want to keep passwords, and make sure to include apps. Allow that to complete.
5. From the iPhone screen in iTunes, hold down Alt (on Mac) and Shift (on Windows) and click on Restore iPhone and point iTunes to the file you downloaded earlier.
6. Allow the restore to take place and when you iPhone reboots it will be running iOS 8.
7. Reload your data and apps by clicking on Restore from this backup and allow the process to complete.
8. Re-enable Find My iPhone.
If you followed the steps then you're handset is back to running iOS 8.0, and the Touch ID and cellular connection should be working again.