Arris SB6183 modem speeds capped

... SB6183 ... speeds up to 686 Mbps download and 131 Mbps upload ... I am only getting 90Mbps download 25 Mbps upload. My plan says 300MBPs.

Comcast rates the SB6183 for use with plan speeds "Up to 373 Mbps". See the "All Compatible Devices" link near the bottom of https://www.xfinity.com/support/devices (at the moment it points to https://assets.xfinity.com/assets/dotcom/projects/cix-4997_compatible-devices/2024.05.17%20Full%20List%20of%20Compatible%20Devices.pdf, but this changes from time to time).

Speeds around 90 Mbps suggest a problem with the network connection, or the presence of a device (switch? network adapter? substandard or defective Ethernet cable?) that is rated for "Fast Ethernet" 10/100 Mbps speeds instead of Gigabit 1000 Mbps speeds.

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