@NikSf153 You will need to connect them like it is on first or second diagram, with Main Deco connecting directly to ONU and switch or Satellite Deco - to second port of Main Deco.

@NikSf153 Below is the picture from the TP-Link document I recommended you. See the last diagram on it, which is crossed out:

@NikSf153 Please, clarify for me, is this how your Main and Satellite Deco connect to the switch:

@TomorrowV Next time it happens, connect laptop to second port of Main Deco, not making any other changes, and see if laptop can ping web sites and have Internet access while Main Deco LED is red....

@iansmcdonald You can configure static IP addresses in Deco mesh, here is how: How to configure Address Reservation on a Deco Remove static IP address setup on devices, they should be requesting IP...

@NikSf153 Start with this TP-Link FAQ: General questions about Ethernet Backhaul feature on your Deco If you have follow up questrions or it is not helpful, feel free to ask.