WS300 is being identified by Tech Support as an Online Profile issue or an account issue. For WS300's that frontline agents are unable to resolve, they are sending these cases to a dedicated back office team that is resolving these issues.
Because of the way Apple Watch is provisioned (ex: extending your wireless number to it), Rogers needs to add a 2nd line to your account, so if for whatever reason your account isn't eligible to have a line added, it can produce any of the W5xx errors, or the WS300 error.
In my case, my WS300 was sent off to the back office team so they could investigate, and they determined they could not allow me to activate my Apple Watch because of a 13-year old Fido account that was in bad standing (debt was written off years ago). I spoke with management and I'm now waiting to find out if an override can be done. It's kind of ridiculous that I've had my current account active for so long, with no issues, in great standing.. and now I cannot have a 10$/month add-on line because of an inactive account someone found from 2006.