@36ty 

Something might be wrong with Satellite Deco, even if Deco app says everything is OK.

Open Satellite Deco page in Deco app and check what it shows under "Connected to:"

Example, with Satellite Deco (Office) connecting to Main Deco (Living Room):

See if Satellite Deco reports it uses both bands, 2.4GHz and 5GHz to connect to Main Deco. If it uses only one band, there may be hardware issue with Deco unit.

  @Alexandre. Thanks for your quick answer! It seems it's connecting to living room with both bands

  @36ty 

That means Satellite Deco can use both bands for devices that try to connect to it.

The next step would be to promote Satellite to Main Deco, swap Deco units and see if problem moves with Deco unit, meaning no devices will now connect to new Main Deco and everything is connected to Satellite Deco.

The process of promoting Satellite to Main Deco is here: Changing the Main Unit for your Mesh Network

When swapping Deco units, take their power supplies with them, too. 

  @Alexandre. Tried to switch main deco several times, but always get a "setting not saved, please try again later."

  @David-TP Hi! I did submit the logs, but not sure if that will help, because I just installed the whole network from scratch and switched the two decos to see if the problem persists. So the logs might not include the previous problem?

However, what I can say after re-installing everything is: the Deco that used to be the satellite, which is now set as "main", worked fine, and devices were all connecting to it. That is, until I disconnected the Decos from power to move them back to their locations. After booting up, the new "main" seems to have lost it's configuration and flashes a blue light. The other one flashes red, because it doesn't get a connection (obviously).

Seems like there is a hardware problem? Or is there a way to re-install a firmware? Thanks!

Update: even after setting up a whole new network and switching the 2 Deco units, I have the same problem as before: all devices are only connecting to "main". What's odd: the satellite unit is still broadcasting its open "Deco_****" SSID, even though the app says it is correctly set up and included in my network.