@Hoffies 

Your network diagram, the way Deco connect to each other with Ethernet cables, does it look like this:

or like this:

  @Hoffies Hello, if you power off Garage, does Kantoor show a cable connection to WoonKamer?

  @yves_b so really strange, rebooting didn't work. Changing to Router Mode gave the same status. 

Only now Garage and Kantoor were switched

Shutting down the Kantoor gave a working ethernet connection to Garage after that powering on Kantoor also gave an ethernet connection. 

So now everything is fixed. However wonder why it happened and when it returns. 

P.S. happened before fixed it in a similar way as now. No idea why it switches to wireless in Acces Point mode

  @Alexandre. 
which one is how it should be done?

  @DavidDe 

In Access Point mode both are correct, but:

1. If Deco mesh has Satellite Deco connected to Main Deco through the switch (the second picture) 

and 

2. Satellite that uses WiFi backhaul is powered off - and another Satellite immediately switches to WiFi backhaul.

Then it is the case that I am familiar with. From OP replies it appears theirs is not the case that I am familiar with, but something else.

--------------

Usually, hardwired Satellite Deco uses WiFi backhaul in one of the following cases:

1. It is not hardwired properly. This does not apply to Access Point mode, but just for the reference I always recommend to check this document: General questions about Ethernet Backhaul feature on your Deco

2. Switch is not compatible or having issues.

3. Ethernet cables have issues. It could be defective Ethernet cable(s) or not up to specs. Low quality cables could be bought even from reputable retailers. If after reboot of Satellites both come back with Ethernet backhaul but later one or both switch to WiFi backhaul, Ethernet cables should be replaced by different ones to see if it helps.

  @Alexandre. Thanks for the tips. 
 

For your information, my uncle was an electrician and was taking spare commercial grade ethernet cables 'Cat 5e'. I know I could have gone for 'Cat6', but anyhow speeds higher than cat 5e support are not available here. Furthermore, once connected, speed is not the issue.

I will keep an eye on the status of the connection. For now it keeps connecting over Ethernet. 

  @Alexandre. 
I just bought 2x2 BE3300, so 4 units in total. I would like to have the advantage of mesch. I have my router and switch in the basement. Are then the 2 pictures from you working, or witch one I should use as a guide? Thanks for your help!

  @Hoffies 

Cat 5E should be enough. I am also using commercial grade Cat 5E cables, I've got them from my friends at very large data center. Cat 5E cables can handle gigabit speeds and are OK for Deco units with gigabit Ethernet ports.

I run my Deco mesh in AP mode as well, and it is set like on the first picture. 

It is possible that switch has issues handling Satellite Deco. I saw report once of a switch, but don't remember its brand and model, which can handle one Satellite Deco but not two or three. If you are confident in Ethernet cables, but issue comes back, consider trying different switch. TP-Link brand unmanaged would be the best choice.

Once, one of my hardwired Satellite Deco dropped out of Ethernet backhaul. It was fine for more than a year and then suddenly not. After trying different things with no avail, I unplugged Ethernet cable from it and plugged back. Nothing else. A year passed since then, that Satellite Deco stays on Ethernet backhaul.

As silly as it may sound, if problem comes back unplug Ethernet cable for problematic Deco from both ends and plug it back. It might help. If not, plug it to a different Ethernet port of Satellite Deco to see if it fixes the issue.