C7800 on Comcast Xfinity: some, but not all, connected devices have internet access

[jtellerelsberg]

I have a Nighthawk X4S, AC3200 Wifi Cable Modem Router, Model C7800. Often, some, but not all, of the devices connected via wifi cannot access internet, even while other devices are accessing internet. This is true on both 2.4 and 5G bands with security passwords, and the open guest band I opened up. It continues despite restarting the devices and turning wifi off at the C7800 then on again.

Channels are set to auto.

Firmware version V3.01.43

Devices involved: Windows 10 laptops, android phones, iPhone, iPad

Thanks in advance for your help. 

[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?

Has a factory reset and setup from scratch been performed since last update?