@MartyZ 

 I am having the same issue with my AC4000. My cell phone no longer sees the 2.4ghz signal. All devices using 2.4 drop off. And I am seeing the signal drop off with the 5ghz signal too. My router is only 4 years old. I'll be watching this topic closely.

Seriously, not a single tp-link tech on these boards???

  @MartyZ 

They are here time to time. Mostly when no one responds to their issue based on what I have seen.

Are you using separate ssids?

try different wifi channels as the channel you are on may be the issue as to many others are on it or you have some other interference such as cordless phones, microwaves, wireless cameras,...

The wider the band you use the more chance you will get interference. 
 

Turn off QoS if you are using it.

Do you have good steady power? 
 

If using a UPS, test it as I have seen issues when the battery goes bad.

All my IOT devices are on the Guest Network, and they are the only ones on the guest network. All my non-IOT devices are running on the main network and they are all either 5 ghz or hard line.

I have tried every combination of  of channels and frequencies. QoS is turned on. I live in a town house so there is interference. But the issue is that when the interference causes disconnects, the router will not allow anything to connect to the 2.4 ghz channel, on either the main or guest network. It appears that the 2.4 ghz signal gets dropped completely and the router firmware thinks it's still working. The only way to fix it is to de-activate then re-activate the 2.4 or reboot the router.

I understand that interference will cause slowdowns and dropped connections, but with this router as soon as it gets a certain amount of interference it completely kills the channel.