Use a switch or hub or second router in between the AT&T supplied router and the My Cloud.
This is a well known and discussed issue that can be found using the forum search feature, magnifying glass icon upper right. Here are several past threads:
https://community.wd.com/t/cant-connect-my-cloud-using-att-uverse-1gig-fiber-service/140412
https://community.wd.com/t/help-setting-up-mycloud/154716
https://community.wd.com/t/mycloud-isnt-recognized-on-new-router/96491
