Constant background download via mobile internet without permission on iPhone 8

On my iPhone 8 every hour (exactly at the same time: 01:24, 02:24, 03:34 etc.) during 24 hours for a few months already 100 or 200 Kb are downloaded via mobile internet. It works like this even at night while I'm sleeping and my phone is connected to our home Wi-Fi. I've already turn all options when mobile internet can be used in the background (e.g. updating of applications, Wi-Fi support during bad connections), but still somehow my phone is switching to mobile internet and downloading something. Mobile operator's support told me that it looks like browser opening, but they can't tell me what website was visited each time. So if I turn off mobile internet on my phone manually this process stops. But as soon as I leave my home and have to turn it on the process immediately continues. McAfee application shows that everything is ok with my phone.

If someone has encountered something similar, please share your experience of solving the problem.