Ethernet port only 100mbps

Is it a Comcast rented  gateway device ? Does the device have an interface that supports gig connections ? Are you using a minimum of a cat5e ethernet cable ?

Hmmm. Perhaps it's malfunctioning. One more thing that you can try before getting it swapped out for another. Try hard resetting it to factory defaults by pressing and holding in the recessed reset button on the rear (or use the WPS button if it doesn't have a reset button hole) for 30 seconds.  You will lose any customized settings, and they will need to be re-configured from scratch. Good luck !

I'm going to escalate your issue to the Comcast corporate employees (The Digital Care Team) who are available to these boards. You should get a reply here in your topic. Good luck !

You don't mention what the slow connection device is but do you have access to the ethernet adapter settings for it?   I ran into a similar situation on an older HP All-in-One that was limited by it's internal wifi card (think it was "n" max) but the ethernet port was rated for 1GB.  It was sitting right next to the router but hadn't been connected via ethernet.   I connected via ethernet and turned off the wifi --- and the speed dropped to 100MBs - slower than the wifi.    I rechecked the specs and both router and computer could do 1GB.    I dug down through Windows menus until I got to the adapter settings and under "advanced" section the "speed and duplex" setting was set to 100MB/full duplex.   No idea how, when, why that would have been done.   I switched it to auto-negotiated and the link speed increased to the expected 1GB.   Something to check if you have access to the deeper adapter settings.