Check your devices and make sure you don't have any mobile devices doing cloud backups, or syncing with ONEDRIVE. Also, if you have amazon alexa or echo/dots try shutting them down for a few days, and you can use the Cox App on your cell phone to check data usage, see if the data usage went down on the days you had them unplugged. Also, if you have a rin doorbell verify it's not sending up a lot of video.
Unfortunately there are a lot of things that can be using data, but if you have a good router, sometimes it will have logs of data usage for everything connected to it.
but if you have a good router, sometimes it will have logs of data usage for everything connected to it.
I have a Asus router that not only breaks down what devices are using what bandwidth but also what apps(services) are using that bandwidth on those devices. I love it, although it does get buggy sometimes, requiring a reboot. Pretty sure all the Asus routers do this, even the cheap ones. See here for a an emulator.