Hi, my iPhone 8 is using up data every hour on the hour, even when I am connected to WiFi. We have two WiFi connections so even if one dropped out the other would kick in. I turned off mobile data overnight and it still did it, although only a few kB. I have background app refresh turned off, I have set every app etc to only update when on WiFi. It is eating my data, please help!
I would take your phone into an apple store and explain your issue. Sounds like a defect in the phone, as per usual with apple devices. My daughter had an issue with her iphone 8 whereby it would not connect to any wifi network, they swapped the phone and all good.
It might also be that you've got a rogue app using cellular data in the background, instead of triggering an available wifi connection. Check for individual app data usage and toggle settings if you notice anything screwy !