I need to establish relay connection on wd my cloud single bay

I am in a problem. Can some one help me regarding the remote access to wd my cloud?
My ISP has provided a private IP for internet service. My router WAN port gets a private external IP ( 172.xx.xxx.xxx). My router is connected with a switch, which is under control of the ISP. So, port forwarding is not possible in the case. I was using remote access with my wd my cloud (Single bay) using ‘relay connection’ for long time. Recently I tried replacing my old Linksys E1200 router with an Asus RT-AC68U. While I changed my router, my wd cloud device stopped remote access any more since “port forwarding” connection was established automatically (non functional in reality!) instead of relay connection. In this situation I went back to my old router without any luck. The old router also does not allow relay connection any more. May be wd my cloud settings have changed. So remote connection stopped. Local access is okay though.

Now, my question is how to set wd my cloud (and the router) so that the ‘relay connection’ would establish again. Thanks in advance. AH.