Nighthawk ac1900 C7000 drop connection, slow download speed with 5G

[Philipthere]

Router: Netgear Nighthawk ac1900 C7000 Provider: Xfinity Performance Plus Internet 500 Mbps Problem: Many devices at home drop connection several times during the day, and the connection is on and off. Speed test from speedtest.net shows from 10 Mbps to 250 Mbps, never above 250 Mbps. I called the technician from Xfinity, one technician came and tested everything, but no problem detected. But when he switched the router to the rental one from Xfinity, the speed went directly to 500 Mbps. I tried to restart the router, reset it to factory setting, change the channel, but nothing worked. Also, the firmware is the latest V1.01.33. Can anyone help me with this issue?

The cable connection is in the attachment. 

THANK YOU

[FURRYe38]

When this drop happens, do you notice with wired and wireless devices. 

When checking speeds are you wired with a PC for speed testing? 

I would open a support ticket here:

https://www.netgear.com/mynetgear/registration/login.aspx

[Philipthere]

Hi, I don't have any wired device. When testing, I use my laptop and iphone. 

[FURRYe38]

What channels are you using? Auto? Try setting manual channel 1, 6 or 11 on 2.4Ghz and any unused channel on 5Ghz.
Any Wifi Neighbors near by? If so, how many?

What security mode are you using? WPA2 and AES is recommended. 

Try enabling WMM.

Test with a wired PC if possible to confirm those speeds are good.

[Philipthere]

I changed the channel for 2.4Ghz and 5Ghz to the channel that only I'm using, and the security mode is WPA2 and AES. There are many Wifi neighbors near by, maybe around 20. 

I find the WMM setting in Advanced ---> advanced setup ---> wireless settings, and both are enabled all the time:

WMM Support (2.4GHz b/g/n) and WMM Support (5GHz a/n/ac)

I wish I could test with a wired PC, but I dont' have any device that can be wired at this time. 

[FURRYe38]

What channel are you currently using on 5Ghz? 

What channels are your nieghbors using? 

Having other wifi neighbors near by can also be problematic. 

Advanced ---> advanced setup ---> wireless settings, is there settings for Power Output of the WiFi radios? Maybe a drop down menu with 100%, 75, 50 and 25%. If there is, I would try the 5Ghz setting on 50% and see if this changes anything. 

[Philipthere]

Unfortunately, there's no such option in the setting. 

The channel I'm using is 124 for 5G, neighbors are using 36, 40, 48, 132, 149, 153, 157, 161

[FURRYe38]

Is channel 44 available on the modem? 124 maybe a channel your device doesn't work well while using.