How to turn cellular data for specific apps back on for iPhone 6 on iOs 9?

How to turn cellular data for specific apps back on for iPhone 6 on iOs 9?
Added (1). I turned a bunch of apps off in cellular data because i was getting close to my max data usage for the month. Now that a new month has started on our plan (on the 22nd) I want to turn the data back on. However when I flip the switch to on for my apps they don't stay on. If I leave and come back the switches default to the off position and my apps won't let me use them without wifi accessibility. I'd rather not go to an Apple store to solve such a small problem. What do I do?

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