If you had used the forum search feature you’d see that setting up a second router behind the first one has been explained several times. Including as recently as five days ago… 
https://community.wd.com/t/network-settings/174360/2
Some other threads where I’ve given brief directions on how to setup a second router behind the first:
https://community.wd.com/t/my-cloud-is-junk/172751/8
https://community.wd.com/t/changed-internet-providers-now-wd-my-cloud-does-not-connect/168186/7
https://community.wd.com/t/cant-connect-my-cloud-using-att-uverse-1gig-fiber-service/140412/30
