Hey @Mariog7,

I do not think that I have seen a similar behavior reported for wired devices before. There really are not too many reasons why your network or node would be disabled from a wired connection being introduced, other than a conflict in addressing occurring. 

Since you are adding a device when the network goes down, I would suspect that it has something to do with the addressing between your devices and network. I would first make sure that you have not set a static IP on the PC, which may conflict with the addresses given by the Decos.

Have you configured an IP reservation for the PC in the Deco App? 

Does the behavior exist when changing nodes or ethernet cables?

Does the behavior only exist when connecting that PC? If so, you should consider resetting the network adapter settings, updating the drivers of your PC's network card, and reconnecting the PC. 

Do you have a network switch that you could try and connect your PCs wired connection through to see if the behavior follows the PC through the switch?

For your ONT, do you have a model you could provide? If it is capable of a wireless connection, the ONT will need to have IP Passthrough enabled.