@Lous 

This is simple and straightforward setup, it should work. Because it does not, I would suggest you try troubleshooting steps to narrow down where the problem actually is.

First of all, keep dynamic IP on PC. With Static IP it won't properly work.

This is the first troubleshooting step I would suggest:

1. Check in Deco app that all Satellite Deco report Ethernet backhaul. This is to verify there is nothing wrong with Ethernet cables connecting them.

2. Check in Deco app if you have any entries in Block List and if you do - remove them all. Block list should be empty for this troubleshooting. If there were entries in it, after removing them try wiring PC to Satellite Deco again.

The next step will depend on two things I need to know from you:

1. Do you run ISP Router in its default router mode or have you configured it to run in modem/bridge mode?

2. Does your PC have WiFi network card, in addition to Ethernet network card? If it does not, perhaps you have WiFi dongle/stick that can be used with that PC?

Hi  @Alexandre, 
thank you for your help.

PC is on dynamic IP at the moment (and the issue persist) but was tested with static IP. Also tested with a different computer booth static and dynamic IP and the issue persisted.

Lets get in the troubleshoot.

1. I suppose that Ethernet backhaul is when you see the little cable between main deco and satellites so yes it shows it for booth satellites. please see the image below

2. There are no devices blocked

Regarding the other questions:
1. the ISP router is in simple router mode but it was tested in bridge mode with dynamic IP in deco app >> more >> Internet Connection
2. I do have an wi-fi dongle also from TP-Link but it doesnt keep the connection for longer than 5 minutes, thats why I' trying to connect it via cable 

:)

  @Lous 

Your Satellite Deco are connected with Ethernet backhaul. They can use Ethernet cables, so we can assume these cables are fine.

Next troubleshooting step.

Have ISP Router in default router mode.

Keep PC with dynamic IP configuration.

Configure Deco mesh to run in Access Point mode. How to set up Deco to work in Access Point mode

This will eliminate all Deco mesh configuration settings that could, at least in theory, impact Internet connectivity. In AP mode, as far as wired devices concerned, each Deco is not much more than 2-port Ethernet switch.

With Deco running in AP mode, wire PC to Deco x20 (room 3) and see if PC has Ethernet and Internet access. To cover all bases, reboot PC after you wire it to Deco.

ISP Route --> Deco x20 (primary, AP mode) --> Deco x20 (room 2) --> Deco x20 (room 3) --> PC

----------

If PC still does not have Ethernet and Internet access, disconnect Deco x20 (room 3) from Ethernet cable and connect PC directly to the end of the Ethernet cable to which Deco x20 (room 3) was connected. You now have:

ISP Route --> Deco x20 (primary, AP mode) --> Deco x20 (room 2) -----------> PC

Reboot PC again and see if it gets Ethernet and Internet access.