This situation took me 3 months to figure out. No fault of my own, updating your iOS is a gift and a curse. Important internet based apps can easily be blocked from using your iPhone’s cellular data. Wifi usually has all apps automatically turned on.
Go to setting and click “cellular.”
2. Once in cellular tab, scroll and slide “green” / on your desired app.
iPhone 8 http://amzn.to/2xawA7D
iPhone 7 http://amzn.to/2yDsLv5
iPhone 6s http://amzn.to/2yxpAWw
iPhone SE http://amzn.to/2xUsnEE