@JoeBlow 

Any help? I now confirm if I turn off 2.4ghz, deco A will not serve any clients. Meaning it is not capable of supporting 5ghz. Is this hardware issue or some configuration to turn on the 5ghz support? 

@JoeBlow 

This is obviously hardware failure. There are no hidden configuration settings to enable/disable 5GHz per Deco unit.

TP-Link Support checks this forum during business days in Asia-Pacific time zones. You may have to wait till Monday to have them reply, but they will tell you the same. If you can return your set and exchange it for new one, just do that. If return window is closed, you'll need to request warranty service.

@Alexandre. 

Thanks for the reply. Yeah I thought so too, and did extensive testing. But strangely, my new Sony A8H model managed to get 5ghz... So this is very troubling since s21 ultra also cannot. Must be some wifi chipset issue that's screwing things up. Tp-link support requested I send back via warranty, but I am still hoping someone in this forum can point out maybe this is something that can be solved by firmware or some software update. 

@JoeBlow 

You need to establish decisively if your X20 unit does or does not broadcast on 5GHz. This is how you can do that.

Install WiFi analyzer on s21, I recommend this one from google app store: https://play.google.com/store/apps/details?id=com.ubnt.usurvey

Turn off 2.4GHz on Main Network. Standing close to X20 unit, turn off/on WiFi on s21 to force smartphone rescan WiFi networks. When smartphone reconnectes to WiFi, open WiFi analyzer app and under Wireless you should see screen similar to the following:

With your home network SSID instead of "Deco 5GHz." Under it, WiFi analyzer will list all nearby nodes broadcasting with that SSID, such as:

There could be MAC addresses instead of Deco node names, but with two nodes it is not hard to figure which is which. In that example, it is obvious I am standing near Deco Node 1 with strongest WiFi 5GHz signal, blue "linked" text means my smartphone is connected to that node, and also there are two other Deco nodes. 

In fact, I have four Deco nodes on my home network, but one is too far and is not visible to my smartphone when it is near Deco Node 1.

@Alexandre. 

Thanks again for the help, this wifiman app indeed good. The problematic deco A is now the main router. The Mac address with c4 is deco A and c7 is the working deco B. I took this screenshot right next to deco A which is at 1st floor. Deco B is on 2nd floor. As you can see the 5ghz from deco A is there but the signal is - 77dBm,kind of strange. The attenna inside faulty? Basically if I turn off 5ghz my device will not connect except for that Sony A8H. Wifiman states A8H using chrome cast? 

Anyway the deco A confirm is faulty but just don't know if there's a way to resolve without returning. Also the signal from 2.4ghz from deco A seems weak despite me standing next to it.