@laplacek Sorry you received a faulty Duet. It does happen from time to time. Yes, boards are tested before they leave the factory, including the WiFi; there's a full test rig. It does look like there wasn't much solder on the WiFi module. I'd guess that it worked in the factory, but maybe movement or thermals cracked one of the joints on its journey around the world.
Let me know if you have any further problems. We will leave the option of a warranty replacement open.
Ian