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 Wi-Fi accessibility. I'd rather not go to an Apple store to solve such a small problem. What do I do?

Jsut go to your setting and then go to nofications a=, then go to cellular Data and turn on all apps that you want

Maybe you missed what they said. They don't stay on. This happens with my phone too. I go back to Cellular Data and turn on the apps I want but the second you leave and go to the app, they go back to being "off" and you can't use cellular data on them.