I too, had the exact same problem–MyCloud 3TB not communicating with my 5268AC AT&T U-verse router. After reading this post, I also purchased the TP-Link switcher referenced here, plugged it in and it instantly worked like a charm!
Mad props to Bennor for the ethernet switch suggestion and cpt_paranoia for recognizing the Autonegotiation issue between WD and AT&T products.
Anybody else having this problem, do yourself a favor and get the ethernet switch. It solved the problem for me immediately.
