CM500 speeds less than advertised

Hi folks,

I purchased a CM500 in August 2018 and just upgraded my internet to Xfinity full-duplex gigabit service. However, running speedtests only returns results that cap out at about 350/20. Comcast support insists that the issue is with the hardware and that they have enabled gigabit support on my account (which should mean 680 down on this modem).

Are there known speed issues with the CM500? How can I test this to determine whether I have a hardware issue?