iPhone using data despite per-app cellular data being disabled
You need to also turn off background updates, which is where the bulk of the data usage is likely occurring.

Settings> General > Background App Refresh > Background App Refresh Off. Of course, you can choose which apps you wish to turn off individually for background refresh instead.

Don't forget that if you are worried about cell data usage, you can also put your phone in Airplane mode, which turns off all radios, but then manually turn on Wifi. This will force your phone to use only Wifi, as the cell radios are off. Simply turn off Airplane mode when you wish to use the cell radio again.