@dianay 

Please try to run a Wi-Fi analyzer on a computer to check if you can see the 5G network of the AX1800, which channel it is broadcasting? You can also try to modify the 5G wireless channel to 36/44, channel width to 40MHz, then confirm.

If the above modification does not help, please try to factory reset the AX1800 by pressing and holding the reset button as long as the LEDs go off then on, then reconfigure it from scratch to check if you can find the default 5G network on any devices.

@Kevin_Z 
 

Nothing I did in particular fixed this problem - one day the 5ghz network randomly started working again, until the other day when I had to unplug the router.

Now, after several months of no issues and no changes to configurations or devices, the 5ghz network drops every few seconds, is unable to be connected to, or if connected there is no internet. The 2.4ghz network is unaffected.

I have tried rebooting the router, hard restarts, connecting using four different devices, rebooting the modem, changing the channel and channel width, running diagnostics. The firmware is up to date. The router has not moved locations and proximity to the router does not change the outcome.

@dianay 

Have you checked my previous suggestions? 

Please try to run a Wi-Fi analyzer on a computer to check if you can see the 5G network of the AX1800, which channel it is broadcasting? You can also try to modify the 5G wireless channel to 36/44, channel width to 40MHz, then confirm.

If the above modification does not help, please try to factory reset the AX1800 by pressing and holding the reset button as long as the LEDs go off then on, then reconfigure it from scratch to check if you can find the default 5G network on any devices.