Nighthawk CM1100, Nighthawk X8 AC5300 Wifi Router Running Half Speed

Upgraded to Cox Gigabit service and majority of the time I am getting 450mb or less but have peaked at 590mb.  Troubleshhoting so far includes verifying the correct/updated firmware is installed in R8500 Router (v1.0.2.130). Cut out router with a direct connect from modem to computer, reboot for all systems, same to no or little change to speed. Cox technician came over, checked cable from pickoff to the home, from home to the modem, all cables good, reconnected and Cox tech connected his tester /modem and it showed 1.1 gig incoming to the home and to the point of connecting to the modem.  So the issue is with the modem. I'm outside of my 90 days so does anyone have any ideas how to attack this issue so I can enjoy the full or close to the speeds that I'm paying for? I am open to any suggestions. The cable I used was straight out of the modem box to the computer so that is correct as well.