There is a known issue with certain AT&T Uverse routers, one that has been well discussed in past threads that can be found using the forum search feature, magnifying glass icon upper right. WD Support even has an Knowledgebase article on it:
https://support.wdc.com/knowledgebase/answer.aspx?ID=15086
The workaround is to buy (if you don’t have one already) a cheap gigabit switch (sometimes found for as little as $10 USD) and put it between the My Cloud and the AT&T Uverse router. Or configure an unused router as a switch.
Just a few past discussions on the AT&T Uverse/My Cloud issue:
https://community.wd.com/t/cant-connect-my-cloud-using-att-uverse-1gig-fiber-service/140412/
https://community.wd.com/t/uverse-wd-cant-autonegotiate-seriously/178538
