A bit late, but for thoroughness:
In the end these are all the places you have to turn Mobile Data off:
1. Settings/Mobile Data/Mobile Data > OFF
Mobile Data Options > Voice > 3G not 4G (ISP says 4G makes cell data connection even with everything else Cell Data related OFF!)
Settings/Mobile Data/WiFi Assist > OFF
2. Settings/Apple ID/iTunes & App Stores/Use Mobile Data > OFF
All Automatic downloads > OFF
3. Settings/iTunes & App Stores/Use Mobile Data > OFF
All Automatic downloads > OFF
Music > OFF
Apps > OFF
Books & Audiobooks > OFF
Updates > OFF
4: Settings/Safari/Use Mobile Data > OFF (Scroll to bottom, under Reading List)
(Removed in iOS 11)
5: Settings/Music/Mobile Data > OFF
Settings/Music/Show Apple Music > OFF
Settings/Music/Genius > OFF
Settings/Music/Home Sharing > Not signed in
6: Settings/TV/Use Mobile Data for Playback > OFF
Settings/TV/Home Sharing > Not signed in
7: Settings/iBooks/Mobile Data > OFF
Settings/iBooks/Location > Never
Settings/iBooks/Sync Bookmarks and Notes > OFF
Settings/iBooks/Sync Collections > OFF
Settings/iBooks/Online Content > OFF
8: Settings/Podcasts/Mobile Data > OFF
Settings/Podcasts/Location > Never
Settings/Podcasts/Background App Refresh > OFF
Settings/Podcasts/Sync Podcasts > OFF
Settings/Podcasts/Only Download on Wi-Fi > ON
Settings/iBooks/Sync Collections > OFF
Settings/iBooks/Online Content > OFF
Settings/Podcasts/Defaults to Manually and OFF
9: Settings/Facebook/Settings/Mobile Data > OFF
Settings/Facebook/Settings/Location > Never
Settings/Facebook/Settings/Background App Refresh > OFF
Settings/Facebook/Settings/Notifications > OFF
Upload Video HD and Photo HD > both OFF
10: List of individual Apps, under Settings/(List of Apps). Turn off all individually:
Location > Never
Notifications > OFF
Background App Refresh > OFF
Mobile Data > OFF
11. Settings/Apple ID, iCloud, iTunes & App Store (top menu)
Settings/Apple ID/iCloud Drive/ > OFF
12. Settings/Apple ID/iCloud Backup > OFF
13. Settings/Mail/Accounts/Fetch New Data > OFF
14: Settings/Photos & Camera/iCloud Photo Library > OFF
Settings/Photos & Camera/My Photo Stream > OFF
Settings/Photos & Camera/iCloud Photo Sharing > OFF
15. Settings/Wallet & Apple Pay > Not activated
16: Settings/iTunes U/Location > Never
Settings/iTunes U/Sync Notes > OFF
17: Settings/Game Center/Nearby Players > OFF
18. Settings/General/Date & Time > updates automatically.
19. Settings/General/Background App Refresh > OFF
20. Settings/Notifications/ > OFF. List of Apps you can turn off Notifications for.
21. Settings/Privacy/Analytics/Share iPhone Analytics > OFF
Settings/Privacy/Analytics/Share iCloud Analytics > OFF
22. Settings/Privacy/Location Services > ON
Settings/Privacy/Location Services/Share My Location > OFF
Settings/Privacy/Location Services/Maps > NEVER
Settings/Privacy/Location Services/System Services/ > List of items to turn off:
Compass Calibration > OFF
Emergency SOS > OFF
Find My iPhone > OFF
HomeKit > OFF
Location-Based Ads > OFF
Location-Based Apple Ads > OFF
Location-Based Suggestions > OFF
Mobile Network Search > OFF
Motion Calibration & Distance > OFF
Setting Time Zone > OFF
Share My Location > OFF
Wi-Fi Networking > OFF
Frequent Locations > OFF
iPhone Analytics > OFF
Popular Near Me > OFF
Routing & Traffic > OFF
