Sorry Koops. There seems to be confusion in my memory.
Like you, I could see the device in DEP, but I couldn't search by serial number.
I couldn't see it right from the'devices' screen, but I could see it by changing to'General -> DEP'
After 2-3 hours elapsed, the devices appeared in the device list, and the serial number was also available for inquiry.
Maybe you also look up normally by now