Internet disconnects randomly and showed "connected, not internet access" with my wifi devices

[Guangguo]

I just got the wifi cabel modem router with comcast service, but that devices which connected to wifi(some connected to the AC1900, and some of them connected to the APs that behind of a cisco switch) always get disconnected from the wifi randomly ("connected, no internet access"), but the system showed that route works fine and the wired network is good too. And the situation would be resolved by itself and come back after sererial minutes, anyway this situation would happen for every devices, but isn't same time.

What can I do to fix it?

[Clancy15]

My modem has been acting strangley for almost 3 months, but once I switched it to the 5ghz option - it seems to have resolve the dropped VPN and tv buffering tremedously.  This is only been a few hours since I switched that up.  My next challenge is trying to figure out how to upgrade the driver s/w for Windows 10 now... seems to not want to let me get to it from the updates page.