@Dr_Mono By "extender" do you mean a guest network on the same router or a different access point?

If it's the latter, then we can be surer that the Asus firmware is what's causing the issue.

@Dr_Mono Yes, I tried the same firmware upgrade, and it did not fix the solution.

The only thing that has partially worked is changing the Asus 2.4 "wireless mode" to "legacy" from "auto."  This apparently limits the transmit rate on 2.4 channels to no more than 54 Mpbs. 

One client is still disconnecting and reconnecting every few seconds. Most of the TP-Link devices are staying connected, but not all.

Also my Ring doorbell and Honeywell thermostats unpredictability disconnect and reconnect anywhere from 10 minutes to 12 hours, where other items stay connected until their leases expire.

@Anduvo This did not fully work for me --- see my comments above.

As another workaround, you can also setup another router in bridge mode broadcasting the same SSID with same password so you don't have to reconfigure your devices. On your Asus router, you'd turn off the 2.4GHz band. 

I did this with an Apple Time Capsule to serve my 2.4GHz devices, while my 5GHz devices continue to connect to my Asus router. I set up some access controls on the Time Capsule to prevent certain dual-band devices, such as my phone, from connecting to it in order to make sure they connect to the Asus router on 5GHz. 
 

Doing it this way using bridge mode, all my DHCP reservations, manual hostnames, VPN, DNS and other settings and customizations on my Asus router continue to work as expected with respect to all the devices connected on my network.