Welcome to the community!
As you are aware, Fido does not currently support cellular services for the Apple Watch. While Apple recently added a new feature with regards to how the Apple Watch may function (ie Family Set-up support), the watches still need specific Apple Watch plans. Generic data-only or Tablet plans will not work.
To my knowledge, none of the flanker brands offer Apple Watch plans either (see here). Unfortunately, I think it's rather unlikely for Fido to offer cellular support for smart watches. In order to keep costs relatively lower, the different flanker levels offer different services (see here). If you would like cellular services for your Apple Watch, I think you would need to switch to one of the parent providers.
Hope this helps 😀