I went over by about 10gb, and had the same message on the xfinity page. I went to chat and confirmed that i was not charged. They really need to add some logic to the web page to take into account the 2 grace periods. I did save the chat log to document what I was told just in case it does show up on the bill.
Also, as far as Firestick data usage. It shows up in the DHCP log as a kindle device, which confused me, and the new version of software on the firestick actually shows the data usage for the month for each app. I didn't find that out until after I had gone over on data, but it did confirm that it was Kodi.
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We were getting warnings about approaching the limit, and I was able to finally figure out what was causing the extra usage. By looking at my router logs, I was able to determine which device was using the most data, which turned out to be the Firestick. We had a Kodi plugin that was running the Apple TV flying screensaver. The flying screensaver shows an HD image of an overhead view of various live scenes that slowly scroll across the screen. It turns out that this screensaver continuosly streams the video from Apple. We had also just purchased a new TV, so when not viewing the Firestick in Kodi, we had left it running in the background. We had been averaging around 400-500 gb a month, except for this month. We stopped the screensaver and the data usage dropped immediately.
So word to the wise. If you have an AppleTV or a device running the AppleTV screensaver, disable it. It uses a tremendous amount of data while running.
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