@Dipal Hi all, I can confirm that this is still unresolved since my original post and seems to be going round the block. I have since got rid of my Deco mesh system and left it in a black box in the shed. I have purchased a Linksys mesh network instead - will keep you guys posted on what it is like in comparison. 

If anyone else has this same issue, then I would recommend to ditch TP-Link, and go with another company too! 

  @Dipal @David-TP I've been having this issue since the last firmware update to my 3 Deco M5s. But when I tap "Detection Failed. Please try again." it then appears to scan again, for only 1 to 1.5 seconds, and returns the result that both channels are optimal and reports the 5GHz channel to be Channel "0". As reported by the clients, It's actually using 5GHz channel 44, and will never ever stray from that despite reboots nor changing its 2.4GHz channel.

I contacted support who eventually funneled me to the RMA department who rejected my request because it's been more than 90 days since purchase.

Pretty disappointed.

  @David-TP Thank you for the advice. While I have you, may I ask if there's any way to backup the Decos' configuration?

From what I see in the app and found searching these forums, it appears there's not, but double-checking. If not, hopefully that's a feature that could come in future firmware, as it's been a relatively standard feature of other router products and would certainly be convenient to customers who set up separate ioT networks, QoS preferences, and client connection preferences. 

EDIT: Second question, I actually purchased 1 unit via eBay as the seller, the other 2 Deco units were purchased from Amazon, also "renewed", approximately 1 year ago. Would they be subject to the same 90 day window, and does Amazon cover them any longer? If not, unfortunately all 3 units are past 90 days.

  @David-TP 

i have exactly the same issue, no more optimisation possible and even with the latest update...

  @WalterH Hi, I can confirm since starting this post I have had a couple of correspondences with the Deco technical support team. All of the correspondences have fizzled out with no solution to the issue. 
 

I have since ditched the Deco mesh system, and gone with another competitor. 
 

hope you all manage to find a solution in the near future!! 
 

best wishes 

 I've been getting the same issue with my 3x M9's for the past year or so, disappointing that I'd have to redo my entire config and Nat rules just to try a system reset that may not even solve it