Netgear Nighthawk - AC2100 - Erratic Speeds

Hi all,

I recently purchased the Netgear Nighthawk AC2100 for my parents. I have an Arris SB6141 and Comcast Xfinity internet(150MPS). I setup the router through the Genie and updated to the latest firmware: 1.2.0.36. I then performed a factor reset on both the modem and router, as recommended. My speed range is across the board. I have gotten as high as 180MPS, right after resetting the router, but this speed slowly degrades to about 60MPS.  I can't figure how to maintain top speeds with this router/modem combination.

Any suggestions on configurations or what to do?

Thanks