The help has been given above. Connect the My Cloud to an Ethernet switch, Ethernet hub, or a second router, then connect that switch/hub/router to the AT&T router. Or contact AT&T and demand they provide you a different brand router.
If using a second router one will have to disable that second router’s DHCP server via the second router’s administration screen and connect the first router to the second router’s networking port (typically there are four of them on the second router, well three because one would connect the My Cloud to one of the four ports), not the second router’s Internet/WAN port.
