Arris S33

Reading through threads a day still have not found an answer. I have an Arris S33 modem. I am paying for 600mbps but only topping out at around 400mbps. (Download speeds of course) Troubleshooting was no help. My coaxial have been redone in the past 2 years by an xfinity tech. Troubleshooting has been done but still no resolve. Anyone got an answer or fix?