Fourfold increase in data usage with no changes in habits

For the past six months or more, we have consistently used less than 500 GB of data per month. Suddenly, in December, our usage apparently shot up to 1,700 GB, with no changes in our habits. I do not believe that this apparent increase is accurate. I have been on the phone with Xfinity customer service representatives at least twice, have done chats and sent messages at least twice, and have tried emailing the address given to me by one representative to escalate this issue, but I feel like I know less about it than when I started. Customer service representatives have told me that they cannot tell what the data usage is coming from because I have my own modem / router and they are not able to view the network traffic by individual device. However, the Xfinity person I chatted with most recently did agree that it would not be practical for us to be using this much data on our current plan.

I have tried numerous things to rule out problems on our end, including reducing the quality settings on all our streaming services, checking for unusual or suspicious devices connected to our network, even resetting our router to factory settings and changing the administrator password. None of this appears to have resolved the issue, as we are still showing high data usage in January. Finally, I agreed to temporarily rent a modem / router device so that my device traffic would be more visible on their end, but this is costing me an extra $19 per month for a problem which I strongly believe is not originating from my network.

Simply put, there is no way that we can actually be using this much data. Nothing in our usage patterns has changed, I have done what I can to rule out common issues, and I have had less than no help from customer support in resolving this issue. I would like to escalate this to someone who can actually discuss the problem and help me find a solution. When this data usage trend is demonstrated to be inaccurate, I would like to return the modem / router device to Comcast and receive a refund for the device rental. I can't get back the hours I have spent chasing this issue, but I would at least like to get the problem resolved.