Steps to take to resolve this issue:
1/ Turn off Wifi
2/ Ensure Mobile (cellular) data is on
3/ Turn iPhone OFF and then ON (re-boot iPhone)
4/ Without turning Wifi on, open an app that requires mobile (cellular) data.
5/ The app will now show on the Settings list with the option to enable or disable mobile (cellular) data
6/ In time all apps that require or use mobile data will be added to the list automatically.