@Kevin_Z  

Thank you for the quick response and follow up questions.

1. When the internet drops on the AX6000, how about the internet LED on it, is it still green or orange? BTW, who is your ISP? Please ensure the modem connection is stable. 

The light does not change, it remains blue. The ethernet connections still work and the router is still receiving an internet signal from my ISP.

2. How many devices are connected to the 2.4GHz and 5GHz network, what are they? Are all of them experiencing the same dropout or just some special or old clients? When they got disconnected, is the wireless network of the AX6000 still broadcasting or the wireless network is no longer available on all other devices as well? 

95% of the clients lost their connection (all of the 2.4GHz and most of the 5GHz). The occassional tablet could still connect, but only intermettently. Both the 2.4 and 5GHz signals were broadcasting, all devices could "see" the networks, but could not log on (iOS devices, Google devices etc...).

If the devices disconnected from the 2.4GHz/5GHz and cannot connect back, what is the error message? I did get the common "incorrect password" or "MAC issue" error message when connecting with iOS devices. The password was not incorrect, and this has happened before.

3. Try to modify the wireless settings on the AX6000 first, such as wireless channel, channel width, which would help in some cases:
https://www.tp-link.com/support/faq/2292/ Thank you. I did not play around with the channels, but can try.

4. Do you need to reboot the AX6000 to recover the connection or do you need to reset it to factory defaults by pressing and holding the reset button on its back panel? I only did a reboot through the app and a power cycle. However, approximately 16 hours later, devices started to find the WiFi signals again. Over the course of a 30 minute window, devices started to come back online (WiFi only) as they could now log on.  Basically, everything went down for 14+ hours and then came back.  But why? Once again, the LAN connections were not impacted, so the router was passing through an internet connection.