@Slog 

If the Deco is set in AP Mode then it doesn't have any control over the routing so I would start with what the main Deco is wired to. You could also disconnect the Deco and plug that same Ethernet cable into the NAS to see if it shows the same thing.

@Tony  Thanks for your suggestions, but the question is why does the Deco 5 show devices as wired to its guest network, which is not possible? The guest network is surely just a wireless concept. I see on other threads that similar behaviour happens on other Deco devices.