Q: Cellular data usage for most apps turns on automatically. How can we turn off cellular data usage for apps?

Hi Krest666,

That is strange that it is coming back on. Do those apps use Push Notifications? If they do then that may be the reason why hey keep turning back on. Take a look at the article below to give more information about apps using cellular data as well as turning them off push notifications Settings > Notifications.

Learn about cellular data settings and usage on your iPhone and iPad (Cellular Model)

http://support.apple.com/en-us/HT201299

Take it easy,

-Norm G.  

Hey Krest666,

The last few things to try is to reset the settings on your iPhone to put that back to their defaults. It will not erase any content but you will have to go in set everything back to how you had it. The last thing to try is to restore your iPhone and set it up as new and test out those apps if the data usage turns back on. 

Reset iPhone settings

http://help.apple.com/iphone/8/#/iph7a2a9399b

Use iTunes to restore your iOS device to factory settings

http://support.apple.com/en-us/HT201252

Take care,

-Norm G.