@fredrick101 Since the complete reboot en switching to Router Mode and back to Acces Point Mode the problem hasn't returned. I think what caused the issues, is that the power went down of the router...

I have the following switch, NETGEAR GS308-300PES netwerk-switch Unmanaged. Deco Units were running also without an issue. One strange which popped up, which was that my wife banking app on her phone...

Connection is like the first picture. Main router, Switch, All Deco Units

I personally prefer the Acces Point Mode option, there was slightly too much interference to create a stable mesh network over WiFi. For me this was a relative easy option, as we have ethernet...

@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...

@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...