A little description:

Yesterday morning my whole network was down when I woke up. After a call with my ISP it appeared my modem was broken.

After replacing the modem and reconnecting all internal devices one by one to not flood the modem with IP requests everything went fine.

Until I want to connect an ethernet cable to my secondary Deco M9 device. The primary Deco M9 device works fine.

When I start up the secondary device it connects with the primary over Wifi and he is able to serve networking and internet connectivity.

However, when I connect the ethernet cable both Deco M9 devices switch to a red LED light and internet is no longer available on the wired and wireless network.

I'm now using the secondary Deco M9 without ethernet cable for about 12hrs and everything seems to be staying online.

Did I misunderstood the installation guidelines several years ago? I was in the understanding that if you connected all Deco M9 devices via cable they would use the wired network as a backhaul and not using Wifi bandwith for that? Or is my secondary Deco M9 broken?

I'm not sure about my hardware version of my Deco M9, I looked it up and the units were purchased in September 2019.

If any additional info is required to help me, please let me know.