High data usage unexplained

We have been receiving emails from Cox saying we are over our limit of data which is more than 1000 GB per month and all used up in 2 weeks. We even received the notice and we were out of town for 2 weeks out of the billing cycle. During that cycle there was more than 100 GB used on certain days when no one was even home.  I've changed my wifi password and my router password. I've had a computer tech come out and check our computers. I bought virus and malware software and downloaded it on my computer. We don't have any gamers or do anything to use anything close to that amount. I even bought a device to monitor our wifi on my phone. There is only one or two devices connected at a time. I have called Cox 3 times and there is still no resolution to this. Other than cancelling my Cox account I don't know what else to do.