@Selik 

I wonder why do you have an exclamation mark on the screenshot: it is usually a sign that something is not right. 

What is firmware version of your Deco M5s?

How many Deco M5 do you have in total? Any of them wired to switches other than Main Deco?

Is it possible that you have more than one DHCP server on your home network?

Can you tell what are these devices connected to switches you don't see in the list: PC, laptop, etc.? Any one of them is portable, that you can carry with you from place to place?

What is your Parent (Main) Deco connected to: router, modem? What is its brand and model name?

  @Alexandre. 

The exclamation mark is there because I have one registered deco, which is not connected to power outlet :)

I have 5 total Decos, none of them use switches except of Main one. I also don't have any DHCP servers except of Deco's one.

The devices I not see are:

• control panel for boiler
• control panel for cameras
• cameras

None of them are portable and they are connected to the network via ethernet & switch.

I assume some of the reserved IPs are used for Deco themselves, but still those numbers do not match

Parent deco is connected directly to optical network unit, nothing is there in between.

  @Alexandre. 

you're right! I managed to get to the cameras and controlling block and indeed they had a static IP assigned. Once I reconfigured them to use DHCP, they showed up in the list and I assigned them static IP through Deco!

Now I need to find a way to do the same for boiler control panel and check there. The only difference in behaviors is that cameras and their control panel weren't working before the change, while boiler panel still works, which is weird. Anyway, much appreciate the help!