Recently I have noticed that my cellular company (TIM, I'm in Brazil) has been charging me for network connections daily. I rarely use cellular network since at home and in my job has wifi and I don't mind being 30 minutes without internet. I called them and they said it's due the device having its preffered network at 4G, not 3G. When I changed the preffered network to 3g, it stopped charging me. They won't retrieve my money because they received a signal so for them there was an internet connection (even though the data received is like 1kb). It's horrible to keep changing it to 3g/4g every time I need to turn it off or on.
But anyway, my device is a Motorola X Play running Android 6.0.1. The question is: does Android send 4g signal even with mobile data turned off? If so, is there a way to make it stop?
Edit: Following some comments, I lsoe signal a lot due the subway and closed places. That could explain it?