Urgent please help us!

we have never used more than 500gb of WiFi now all the sudden with month we have used 1.4TB witch exceeded our limit. I called Xfinity and everyone kept transferring me and every single person tried to upsell me with unlimited Wi-Fi witch is an extra $30 a month making the bill almost $100 Instead of them acknowledging and listening to our concern that we have not used this much Wi-Fi because we are barely home. They decided to all try to upsell me. Even with all of our devices turned off our Wi-Fi goes up 200 GB overnight. Yes, we have changed the passwords and the name and no, it has not helped. We have bought a PC that we thought was the problem, but even with an unplugged, the Wi-Fi still shoots up. No one has been listening to our concerns and no one has helped us with this issue. Even in our sleep it goes up 200GB in 1 night with nothing on and I have told them this. All we were told is it must be correct but it is not because we have not been home to even use this much. Any ideas please help us! We cannot afford the charges to come if this is not solved