Nighthawk router blowing through data

Recently got this as a refurbished from Amazon. At my home I'm lucky to get a 15 mbps connection with a WeBoost whole-home amplifier. I was shocked to get notifications that I had blown through my monthly data and new data was being added. Apparently I had gone through 3 GB of data in just 15 minutes.....and I hadn't even turned on my computer, phone or anything. I verified phone was not on Wifi. Laptop was in sleep mode. The data kept streaming and notification warnings kept popping up so I eventually just shut off the router to keep from incurring more overage charges.

Any idea what is happening?