@Ssal 

Hi, 
It's hard to determine what caused the Deco port to stop working. You may check the following points for troubleshooting.
  1. Please confirm whether the main Deco is powered by the original power adapter and the cord that comes with the Deco nodes.
  2. You may change the Ethernet cable between the front router and the main Deco to see if the problematic port works after trying a different cable.

If the problem remains, you can contact your retailer or the support team to check if your Deco is under warranty.

Best Regards