Data usage spikes

Good Luck! The same thing is currently happening with my parents! They've been with COX for 20+ years. Decided to finally get rid of their telephone and TV and have strictly internet which they moved over to the "Panoramic WiFi Gateway". NEVER had any sort of overage before and now all of a sudden they do. They've been using mostly antenna TV, very little streaming, so that's not the issue. They were even out of town for over a month during which time they had 6 days of what I would consider "normal" usage, 3-6GB but all others were WILDLY out of line, like 35-75GB. All when no one was home to use the WiFi. I've torn apart their entire network. Changed all logins and passwords, unplugged the "Gateway" for a full day and nothing works. Still most days are coming in at 50-60GB, which from what they use their internet for, there is NO CHANCE. All COX continues to do is tell them to spend more money and upgrade to Unlimited Data. Unacceptable COX! Stealing money from elderly folks on a fixed income!