Data Over usage

In recent months our data usage has spiked to the point of having to pay extra for data. I called Cox support and all I got was the suggestion to pay $50 for unlimited data or as a "courtesy" pay $30 for some additional amount of data (I forgot how much data that was).

Our usage habits have not changed, since the problem started, we are being more cautious about how we use data, this is stressful. 

We turn Wi-Fi off in our phones and turned on the setting that prevents apps from turning on Wi-Fi automatically. We do not own game consoles that would consume data incognito, etc. etc. 

The kicker came when we went on vacation for 10 days, turned off every computer in the house and pulled the power on the modem, guess what? we "used" data. 

The tech at Cox could not care less that I mentioned we would have no choice but to look for an alternative solution, he offered to transfer me to the department that would handle the cancellation.

Searching the internet, I see this is not an isolated issue and somewhat widespread. 

Have any of you experienced similar issues?

Mike.