I switched off 4g (LTE) in settings on my iPhone 12 pro max several months ago (it was suggested by someone either here, or at apple forums) and that way, when phone stays on 3G network, it consumes only 2-4 % overnight. Lately I wanted to try it on 4G, thinking may be it's fixed already, and switched it back on, and voila, overnight it went from 19 % to 2 %. ..