Some are having trouble with the newer AT&T Uverse routers. The workaround at the moment until AT&T pushes new firmware to their routers to fix the issue (if possible) is to put a switch or hub in between the AT&T router and the My Cloud. See the following threads for further discussion.
https://community.wd.com/t/cant-connect-my-cloud-using-att-uverse-1gig-fiber-service/140412
https://community.wd.com/t/cant-connect-to-u-verse-help/150157
