Paid for 1.1 Gbps, only getting ~135Mbps??

I have an XB8 Modem (renting from Comcast) plugged directly into the Coax cable going out to the street, no amps or splitters in the way. I've tested from multiple sites on any device that can hit a web browser and I'm seeing speed ranges from 90-150Mbps. I am standing directly next to the device with my phone in hand and still not getting over 150 Mbps. I have reset the modem multiple times, both using the "Restart" option in the GUI and hard rebooting for 15 sec, 15 min, and now 24 hours. None of these have resolved the issues, and none change the behavior of the issue.

After calling support, I was on the phone for about an hour while the technician ran SIX diagnostic tests. Each time a diagnostic was run the speed improved marginally, but it topped out at about 500Mbps which isn't even HALF of what I'm paying for. Even while I was on the phone with support, I could see the speeds dropping back down to 150-200Mbps within 20 minutes. The technician on the phone mentioned that she did not see any issues reaching my device: No packet drops, no latency, nothing that stood out as unordinary.

What's going on here? Do I need a new XB8 installed?
If nothing is "wrong" why am I not getting the "right" speeds that I'm paying for?