Why is my download speed is locked at maximum 15 Mbps when my plan is for 120 Mbps?

I've been trying this over a direct-wired connection to the modem and it's been consistent over several weeks, and across different days/times/devices.  It always runs right up to 15, and then stops.

I've got a CGM4331COM modem that was just installed. I'm paying for the highest internet plan plan and it seems like a waste of money for the speed I'm getting.

Can I get a someone from Comcast to try and resolve this issue?