@Moshe-Steiner 

I have seen this before and sadly there is not a lot you can do about it, its something that is out of your hands.. 

When you run the Network Optimisation it looks at the channels in your airspace to see which ones are free / lighter used and then switches to that channel, the warning you get is as you describe "The current wifi channel is congested"

The problem with this is that its a no win scenario.   Most ISP routers will automatically change the channel when they think a better channel is available, therefore each time you scan for congestion the airspace has changed since you last scanned.. if that make sense?

Say you have 2x neighbours on channels  36 and 40, you are on 36.    Deco scans and finds that 48 is free, says you have "congestion" and swops you to 48.  Bingo Job Done.. however the next day when you are at work and not using channel 48 much, one of your neighbours ISP routers sees that 48 is very light and decides that's a better channel for it to use so swops over.  

You come home from work and rescan, back to "congested wifi", the cycle starts again.   Honestly, unless you are suffering from bad performance this will be a never ending cycle.  Personally, scan once and leave it, only rescan if you need to!

Seen this mentioned a few times with Deco and Aero, its a common enough question.    If you have Android use something like WiFi Analyser to scan and see what channel you are on (Deco won't tell you).  Its likely all the channels (36  40  44  48) are in use by someone so its a no win scenario

Hope that helps! 

  @Philbert 

Thanks for the detailed answer!

I will check with some wifi analyzer....