What is sucking up my mobile data on my iPhone?

I'm new to iPhone, I came from android. And simply turning off "Mobile data" is not working. The solution that works on android by simply turning off "Mobile data" and "Data roaming" is clearly not enough on IOS. There's still something using up my phone balance, I Just don't know what... any help please?
What is sucking up my mobile data on my iPhone?
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