iPhone using data despite per-app cellular data being disabled
Solution 1:
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.
Solution 2:
Going off that Reddit post more than your question but...
Before reading Apple's documentation I always assumed the spinning wheel was started and stopped by the operating system whenever there is network activity. But that isn't the case. It is up to app developers to choose to start and stop it by setting the networkActivityIndicatorVisible
to YES to start it and then NO to stop it.
So, in theory, seeing that activity indicator might not actually mean there is network activity. It could be that an application has started it but for some reason not stopped it.
If the perceived problem is that the activity indicator is spinning constantly, the only way I can think to test this is to turn the phone off completely, then turn it back on and see whether the activity indicator has started again. If it stays quiet or only appears sporadically my guess would be that it appearing constantly is because an application started it but failed to stop it.
Solution 3:
If you want your phone to use NO cellular data, including push notifications, go to Settings -> Cellular and turn Cellular Data to OFF. That should allow you to make and receive phone calls and SMS messages (although iMessage will be disabled).