I was just browsing around http://my.t-mobile.com and under Profile-->Multiple Devices, I see that it's an option. But, I don't see a way to add anything (I don't have a wearable).
You may need to contact Customer Care or T-Force to get the device added properly. I'd suggest T-Force -- they're available via Social Media: Contact Us
I just found out after numerous attempts to solve the problem. The last time I visited the T-mobile retail store. A woman assisted me and noticed the SIM card # did not match the one on the watch. Apparently the embedded SIM card was already activated after being scanned without the package being open, the previous buyer's transaction was canceled thus activating an unused device. They gladly removed the device and replaced it with a fresh new one. All of my settings are now smooth and working properly! Thank you T-Mobile!