I have a 1000Mbps plan, and a CM1200 modem which is configured correctly. I have reviewed all the information you have posted on tips for getting to gig speeds. My downstream power ranges from -2dBmv to +2dBmv over 32 channels with SNR between 40 to 42 dB. In upstream, 4 of my 8 channels are locked, and power is between 40-42dBmv. for OFDM, 1 of 2 channels is locked, with a power of -1.3dBmv with SNR of 39.9dB and 100723119 unerrored, 2 correctable and 0 uncorrectable codewords. There is no upstream OFDMA channel locked (of the 2 available
Just to give a background, I am well versed with most IP protocols and in real life am a IT & software dev. I also run my own router behind the CM1200, and use a <1year hardware with openwrt on it. I have used this exact setup at other ISP locations and I have seen the WAN port provide up > 500mbps measured with standard tools like iperf (https://iperf.fr/ ).
All in all I get like 50Mbps/35Mbps in the dead of the night like 2am, and in the day during normal business hours this is like 2-5Mbps/1Mbps and never over 10Mbps/2Mbps.
I was wondering if it is even possible to get 100Mbps (forget the 1000Mbps).
I am wondering if the coaxial cable in the building is the issue. Unless you ran the cable from the outside to the inside yourself, I would be wondering if that was the issue. I have seen some strange stuff.
I would connect the modem to the actual outside line and directly into a laptop and see what you are getting there. If you still have issues you know it is either Comcast or the modem. If you don't have an issue then it is probably a wiring issue.
I posted a bunch of numbers which all seem to be exactly where Xfinity expects them to be, if there is a wiring problem then my understandting is that those numbers would differ or point out the problem.
Also FYI, the modem is connected to the cable at the first tap into the house, also its the brand new cable which was installed by the Xfinity tech.