slow upstream speed with SVG2482AC cablemodem

Speedtest consistently shows upstream speeds of approx 6Mbps ever since I switched to new cable modem Arris SVG2482AC (Docsis 3.0).   The previous cable modem (XB6 aka CGM4140COM, also Docsis 3.0) usually reached around 39 Mbps upstream.  I have gigabit service.   Downstream speeds are fine, i.e. they range from 400 to 700 Mbps, similar to previous modem.  Yes, I know that Comcast clearly states that they limit the downstream rate to 856Mbps.  My problem is only with the upstream speeds.  Note that the Comcast web page ( https://www.xfinity.com/support/devices/#details) that shows how the SVG2482 will be configured shows that it will use 24 chls downstream and 8 channels upstream.  However, what I see is that only 4 channels are provisioned for upstream and they are DOCSIS 2.0 channels using 64QAM.  Problem.  This must be a mistake by Comcast on the provisioning of the modem.  I have had the modem reset twice without any change.   How does one get Comcast to correct the provisioning of this cable modem for this internet speed plan?   The effect of this glitch is to handicap customer-owned cable modem;  something the FCC would be interested in.