Verizon Wireless Mobile Broadband device miscalculating usage STILL even after a replacement device! Wallingford Connecticut

I have been a customer since February of 2013 and I purchased the mobile broadband device from Best Buy via a special offer.  Within the first 2 months of service, the standard plan that i purchased of 4GB was apparently not enough even though there were only 4 devices connected, and my bill went immediately over before the end of the billing cycle; I called customer service and they did a data analysis and apparently 4 or so IT peoplel down the line, agreed that there was something not right in calculating the data. I even had one IT person say to me that he doesn't see the usage matching the calculation!  I kept asking that the overage be taken off of my account.  They would not...and instead suggested that i change the data plan to 10G.  I agreed, but i told them that something is wrong with the mobile broadban device.  They said that they couldn't tell that, and that there was nothinh they could do about that but lets see how raising the data plan would affect my next bill.  The next bill, again, went over by a whole lot and I continued to asked for the overage charges to be taken off of my account but this time I insisted that somthing was wrong with the device.  After enduring the humiliation of "IT 101 tutorials" on how data is calculated, even after they still couldn't see the usage coming from all of my devies, I again insisted that they change the mobile device or i would cancel my service.  I finally got them to send me a replacement device and all was well until 1 month ago when the usage and overage went through the roof again!  I got fed up and called to let them know that I wanted to cancel the service because i think somone is "piggybacking" off of my wifi! I would turn it off and even unplug it, and I was still getting usage calculated!!1 How is that possible?!  Beware of Verizon JETPACK!  I believe this device is defective, and I think that billing tacks on additional usage charges even if they are not calculated correctly.  They STILL wanted to charge me the early termination fee!