Bennor and Cat0w correctly point out that WD cloud services do indeed solve this problem by informing WD just where on the planet the NAS box is located (by periodically sending WD the WAN IP address). The only downside to this is that as a result you have to use WD connections (either the APP or mycloud.com) to access the system.
What the OP wants is a way to have a service link his dynamic router WAN IP address in a way that he can access the NAS WITHOUT using the WD servers. In this day and age. . .a wise choice. Torch1 is on the right track. I access my system this way; using a Open-VPN to encrypt the connection to my home system.
I have an ASUS router. ASUS offers a DDNS service that links your router to a personal domain name. So, if I type the appropriate webaddress (something.asus.com); I can use this to access my NAS (on port something.asus.com:7001; assuming I have port 7001 forwarded to an appropriate port on the NAS) www.noip.com is performing the same function.
The key is that you need a router that supports an embedded DDNS client. I did a quick check on the noip.com support page. . . . .ASUS, Netgeard, and TP-Link seem to have routers on the list of supported equipment. So. . . if you have a supporter router - - - you can make this work and you won’t need a PC running on your network to make the magic happen.
There are instructions here: https://www.noip.com/support/knowledgebase/how-to-configure-ddns-in-router/