600Mbps Service, getting 100Mbps at Modem

I have the 600Mbps Service, but only getting 100Mbps at the Modem.

I had an Arris SB6190 with Asus AC1900. I only managed to get 100Mbps over ethernet (modem/router) and 2.4 wifi.  On 5G, I mangage to get 450Mbps.  

Then upgraded to SB8200 and Asus AC2900.  nothing changed and same speeds remained.  I called comcast, rebooted the modems/routers & no change.  I also used different ethernet cables, no change.  Any thoughts?