@Stitch1970 

You have networking setup which offers great flexibility in how you can connect wired equipment to it. You didn't do anything wrong and it should work, but for some reason it does not.

If you don't want to bother with troubleshooting, just connect WD MyBook Live to one of four Ethernet ports of your ISP router, hopefully you have at least one available. That will make it bypass Decos completely. Whatever problem is, most likely it would be solved by doing that.

If you want to troubleshoot, the initial basic steps would be, assuming you have WD Mybook wired to Deco:

1. Reboot (power off/on) WD Mybook. Check after it came online if you see WD MyBook in the list of Network Devices in Deco app;

2. If you do, and it is listed as Active, note its IP address from Deco app;

3. Try pinging that IP address from laptop, and try accessing WD MyBook folders using that IP address. In File Manager, try accessing \\IP address\public 

If IP address is for example 192.168.10.11, you'll be trying to open \\192.168.10.11\public networking folder.

4. Launch the MyBook Live Dashboard. Using same sample IP address from previous step, that would be opening in browser http://192.168.10.11

Of course, replace that sample IP address by actual IP address you've got from step 2.

Also, if you check back panel of WB MyBook, you'll see if it successfully connected to the network and at what negotiated speed (see MyBook manual for specifics).

My suspicions would be either you set MyBook storage device with static IP that somehow now conflicts with Decos (one of them got same IP, who knows), or WD MyBook got new, and different, IP address from router after it were connected through Deco but your laptop tries using old MyBook IP address to access it.