I'm having the same issue. I'm with AT*T and have a PACE 5268AC and a 4 TB My Cloud Model: WDBCTL0040HWT-NESN. The issue seems to be the router. It fails to even recognize the My Cloud device. I'm going to try a work around later with purchasing a cheap router to plug the My Cloud into then plug that into the PACE 5268AC router. If that doesn't work I'm switching Internet providers. I didn't pay $200+ for a device to not work to its fullest capacity, and this is obviously not the My Cloud. Yes I've tested it and I'm connected to it right now. It's not a Cat 5 cable issue either. The router refuses to see the My Cloud device at all (no lights on the Ethernet port when connected, and I've tired all 4 ports).
By connected to it right now I mean I'm connected via my laptop using direct Cat 5. This unfortunately does not allow for me to access the My Cloud for data backup using the Internet, which is the whole reason I bought the device.
I went and purchased a NETGEAR 5-Port Gigabit Ethernet Switch (Model # GS605). I got it to work after fussing with it. I spoke to both AT&T, their 3rd party tech support (who wanted to charge me for further tech support, so I hung up on them), and Western Digital Tech support. Here's what I've gathered. The PACE 5268AC does not support UPNP at all (something dealing with it as a security issue - Google it another forum has that info). As a work around I got the Netgear switch (about $40). I connected my WD My Cloud to the switch then the switch to my Port 3 of the PACE 5268AC router (using Cat 5 cables). I factory reset the router.
Once it launched I waited until it saw the WD My Cloud, which I could access if I entered the IP address for the WD Device. I still had an issue with not being able to access over the Internet. Had an issue with the WD My Cloud device refusing to let go of it's old IP Address in the WD software when I tried to .
So I rebooted both my PACE 5268AC router and the WD My Cloud. Waited for it to come back up, and it worked! I now have access via the Internet to my WD My Cloud. I've connected it to my smart phone (android) using "Get a Code" button under Cloud Access. You may need to send yourself another email from the add users. For some reason the original email address I launched / registered with refused to work and did not see the My Cloud Device via the Internet. I had to re-add that email and new user name. It works now. Overall this was a ridiculous endeavor due to PACE 5268AC routers (AT&T) not supporting UPNP. Their tech support should know this.
This isn't ideal, but at least my device is now working on the Internet. I can back up my smart phone, and transfer files to the MY Cloud device when I'm away from home out in the field on my laptop.