only half of download channels in use and slow download speeds with gigabit service and new docsis 3.1 modem

I have been having problems with only getting 200-400 Mbps download speeds for about a year, even though I have 1200 Mbps service.  I recently upgraded from a CAX30 to a G36 to get me into Docsis 3.1, but nothing has changed.  Still slow (recently 200-240 Mbps speedtests from Wi-Fi connected and wired connection to the gateway.  I am seeing this, though (which I did not see with the CAX30): half of the downstream QAM channels show no traffic at all.