'Use mobile data for' toggle changes won't save on iPhone 6, iOS 9.0 When I change an app's setting to use mobile data (or not), the changes aren't saved when I exit settings. It's infuriating as, for example, I can't use WeChat at all unless I'm on wifi.

Elsewhere on this forum it's suggested that the only way to rectify this is to restore not from backup. Is there really no other way? And if I do need to restore not from backup, how do I do it? And what will the consequences be - e.g. lost contacts, etc?

Many thanks for any help.
As a troubleshooting step, I often will back up an iPhone to iTunes and then the cloud (just to be safe/sure) and then erase all content and settings.

Settings app > General > Reset

At that point, set up the phone as new and don't sign in to iCloud. I opt in to all the other settings and then go check the setting that was "stuck"

At this point you will know if the setting works or the phone needs the OS restored. If so, do the restore or seek service. When that's done, you can either decide to erase all content and settings to then restore one backup or if you want to just start downloading your apps, signing into iCloud and moving forward with a base that works.