Paying for 600 Mbps down on a pro plan and only getting 90 Mbps via ETHERNET connection...

Registered with Comcast for the first time last week with a plan that is supposed to get me 600 Mbps download and 50 Mbps upload. I noticed that the speed was slow and that my internet cut out frequenctly. Decided to run a test today while connected via ethernet. I am receiving 96 Mbps down and 20 up when connected via ETHERNET. OVER FIVE TIMES SLOWER THAN ADVERTISED.Can't help but feel like I'm getting scammed.

Has anyone else had the same experience and was Comcast able to fix it?

Edit: I am also renting a modem from Comcast