Arris G34 Wi-Fi Cable Modem - the modem isn’t optimized for the plan you selected (download 1100 MB)

... Why it cannot support my plan for 1100MB download speed? 

Manufacturers tend to exaggerate the speeds of the devices they sell, for example, by adding the Ethernet speed to each of the Wifi channel speeds, or by adding the 1 Gbps speed of each of its 4 Ethernet channels together.

Comcast/Xfinity rates the G34 as compatible with internet plan speeds "Up to 934 Mbps", so it's not compatible with a 1100 Mb plan. See the "Compatible modems for enhanced [upload] speeds" list by following the "See all modems" link on https://www.xfinity.com/support/internet/customerowned.

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