Can't modify selection of "use cellular data for" for some apps in iPhone 6 after update to iOS 9 After update to iOS 9, for many of the apps in my iPhone 6, the "cellular data" selection is locked in "Settings/Cellular/Use Cellular Data For" or in the corresponding "Settings/[App Name]" entry. The toggle can be flipped but the new value is not stored when exiting Settings. If the value reverts to off, the app can't access the cellular data network. The apps with the value reverting to on, can. Cellular Data and Data Roaming are on. All apps have the most recent updates.

I've tried toggling "Airplane Mode," "Cellular Data", turning off the iPhone, resetting "All Settings," loading a previous back-up, and restoring the phone. Nothing seems to work. 

Please help!
There's been a bug with iOS since iOS 7 where the CellularData.db can become corrupted. Once the file has a problem, the Settings app can't save changes to it and there's no easy way to wipe just that file and start over.

This file is saved as part of the backup process and then restored, still broken, when you restore from backup. 

Now that a jailbreak is available for iOS 9, you can manually delete the CellularData.db to reset it which avoids the need to restore the device (and not restore from backup) which is an alternative.