Can't modify selection of "use cellular data for" for some apps in iPhone 6 after update to iOS 9
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.