" He then checked the line (from the splitter) to ther modem/wall port."
So the problem is the line that goes from the panel box to the room the modem is in? if so, why not get that repaired? Looks like that ingress is leaking over into the 837Mhz range. What he probably meant is it isn't enough to stop a D3 modem from getting online, but its well within the operating parameters I can see the Technician saying he can't fix it if it's wall fished, but I don't see how he can say it's not a problem at all. Changing the modem is a "cross your fingers" kind of fix, and not one I suggest. I would focus on either repairing the line that goes from Master bedroom to where modem is, or investigate the possibility of moving the modem into the master bedroom. Or just hope what ever is causing the ingress to go away, but I doubt that will happen.