@Romren 

This may not be the Deco but rather your internet connection that is throwing the warning, what speed is your internet connection?   35 devices is not an insignificant amount, and I would expect most APs to detect the channel as heavy utilised with 35 devices on it.

@Philbert 

Have a BT modem/router and managed fine. I bought the Deco 2 days ago because I got told it managed the number of devices better.
There is a large number of devices but most of them do not stream anything. 
I have 60 Mbps connection and generally I don't go over 20 unless I'm downloading a game. 
I also used the Deco Lab gadget to test the network speed and latency and it was all excellent as reported by the app. 
 

The issue is more obvious streaming audio, I assume there is less buffering for that. The audio is severely impacted, never experienced this on the BT router . 
The Deco happens to sit next to the audio streaming devices too. 
 

The only indication that there is something wrong is the "channel congested" message in the App but reading around it seems that everybody gets that so maybe it's a red herring.