@Pere-Pozo-Miro You are probably using a dock for network connection, right? Some of the docks do a pass through of the device’s MAC address or hand out the dock’s MAC address either.
Which exact model of dock do you use?
How do you register the device? Through the PXE menu or manually in the web UI? What MAC do you see in the web UI for it after registration? Please post the full MAC address or at least the first three blocks here in the forums so we know which vendor it belongs to.