I have Netgear CM1150v which has never given upstream speeds over 18 mbps for contract 20mbps

I have Netgear CM1150v which has never given upstream speeds over 18 mbps for contracted speed of 20 mbps. Recently Xfinity upgraded the upload speed to 100 mbps and I am still getting 11 mbps. The issue was referred to Netgear support with event logs and channel power levels. They suggested T3 errors and so Xfinity technician was scheduled to check it. Technician checked everything and in the end checked the speed from the cable where it connects to the modem. Upload speed capability was over 400 mbps and download over 2000 mbps. Still Netgear CM 1150v refuses to give these speeds. Xfinity technician then showed only 4 upstream channels are being locked (which support only 22 mbps) and used in the modem. Xfinily has a fifth channel open for upload that supports over 400 mbps that the techniciam showed on his tool. Can Netgear lock and use this channel or is this cable modem useless for getting better upload speeds? This is a DOCSIS3.1 cable modem and poor upstream speeds are disappointing. Any suggestions for resolution?