Not getting proper Cable Modem speeds

I own an Arris S33 cable modem and have upgraded to the 2Gb plan with Xfinity. After doing a self install, I was only able to get 1.4G down and 40 up. I called support and they sent someone out to check my cable line. I walked the installer through my setup: I am in a rural neighborhood, and I have 1 line coming into my house that goes directly to the cable modem. I have no other cable devices. There is one straight-through connector outside my house and no splitters. I am connected directly to the tap, according to the installer. As a test, the installer put one of the comcast cable modem/router combos in place of my modem and experienced the same speed cap. He was eventually able to get the support group to assign a manual profile to the xfinity modem and get it to do 2Gb down. I asked him to put my modem back in play and repeat the process. We ended up still capped at 1.4G on my modem, with the support group saying they did not have access to my modem, so they could not manually assign a boot file. As far as I know, that’s what DOCSIS is for… once I place the modem on their network, it follows whatever instructions they send it. I did provide them the errors my modem was noting, namely that the bootfile they were providing was invalid, and that the signal on the upload side was too far out of spec. The installer left it with me saying that I needed to contact Arris because the modem was likely bad. I was charged $100 for the “Professional Install”. Upon filing a support ticket with Arris, they determined that the upload SNR was 10db below the minimum acceptable threshold. I was getting 40db and the minimum required is 50db. Since the technician said the signal is fine, how do I get Xfinity to correct my upload SNR? This modem is certified to work with Comcast.