T25 Modem not getting Gigabit speed

Hello,

I signed up for Gigabit plan and should get 1200 MBPS speed. However I am not able to cross 935 download speed. Upload seems to be fine clocking at 39. Comcast is saying contact Arris to change configuration. Arris is saying there is nothing to change. Anybody has same modem and getting gigabit speed from Xfinity? Thanks