I found a couple free iOS apps that track usage, Data Monitor and My Data Manager. the second uses a VPN connection. The reported usage varies a bit, but considering how little cellular data I use, I don't know how much it would affect your results. FWIW, I think the Xfinity app reads 0.0GB until usage goes over 1MB, then it shows 0.1GB. As you said that is not very useful to watch the 1-99MB window. I usually check cellular data usage in Settings/Cellular, which also gives a breakdown of usage per app and system service.
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