Nighthawk ac1900 c7100v speed

[robstlaurent]

I recencently upgraded my internet speed to 600 Mbps (from 300). I have a Nighthawk AC1900 C7100V, which says it should be able to accommodate speeds up to 960 Mbps. However, speed tests on my computer are topping out just over 200 Mbps when I only have one device on and and nearby the modern router.

Does this Nighthawk model not really work with 600 Mbps speed, despite what the specs say? Is there something I need to update? 

Alternitive is renting the Comcast Xfinity modem router, which I prefer not to do since I already own the Nighthawk.

[FURRYe38]

Did you power OFF the modem for 1 minute then back ON after the ISP upgraded your speed? Usually this is needed.

Have the ISP check the signal and line quality UP to the modem. 
Be sure there are no coax cable line splitters in the between the modem and ISP service box. 
Be sure your using good quality RG6 coax cable up to the modem.