Are you streaming video on a TV (Netflix, Disny+, Hulu, Amazon Video, etc)? That counts against your data limit. The normal channels in your package do not, just the data consumed while streaming video. Some home security systems consume data if you have the video feeds to a monitor in your home. I understand that you have unplugged your router for extended periods; that would indicate that whatever is using data is connected to the modem (assuming you have seperate router/modem).