Unstable and low speed

I have a NETGEAR Nighthawk CAX30-100NAR Router. This router is on the recommended list by Xfinity. I'm on a 1000 Mbps plan; the router should give me speeds in the range of 800+. On average, the real speed is around 100-150. Sometimes at night, it rises to 500-600. But it happens very rarely. I have tried all the recommendations. The speed is consistently much lower than it should be (less than 5-6 times). What should I do?