C7800 Wifi speed is slow

I just bought C7800 several days ago, because I need DOCSIS 3.1 model to support gig speed of Comcast. 

Then I notice the speed of 5G wifi only have 400Mbits and it should be near 900Mbits to 1000Mbits. I call xfinity and they send a guy come and test wired on their test device. The speed is 900+Mb.

I have tried:

1. reset device.

2. change wireless channel to other number instead of 153.

None of them can make my wireless speed faster than 400Mbs.

Test method: speedtest.net and xfinity.speedtest.com.

Firmware Version: V3.01.43

Does anyone have solutions of this issue?