@jacktiew 

On picture you shared, how many of these are your Deco units broadcasting on same channel? 

For WiFi channels congestion reporting try WiFiMan app from Ubiquiti, it can show WiFi networks in more meaningful way:

  @Alexandre. 

This is in a good case. For the worst case you can refer to the pic in the #1 post.

  @jacktiew 

The screenshot in the #1 post is not helpful. All your Deco nodes broadcast on the same channel, of course it will appear congested. Even if you can change Deco WiFi channel manually, the new channel will appear congested, because now all Deco nodes broadcast on it, and in close proximity to your smartphone.

Answering your original question: TP-Link declined feature request to change WiFi channel manually. Here is more detailed explanation, which is already 1.5 years old.

Do not expect to see that feature in Deco app any time soon.