I went into a Videotron store and finally got a straight answer.
I was told that Videotron’s native LTE network runs on a different frequency than most networks.
The Apple Watch does not support that frequency in the current series 3 and 4 iteration. They believe the next series will support it, i.e.likely late 2019.
Although Videotron “shares” some of the Rogers network, and, in theory, the Apple Watch could work on that network, Videotron’s working arrangement with Rogers does not allow for 100% reliability on the Rogers network.
Per the employee, there are no plans to work the Rogers agreement.
Buying a series 3 or 4 with LTE is a waste of the LTE adder if you are on Videotron per this employee.
Strange though, when I check the Videotron frequencies and the watch supported frequencies..they seem to match. Go figure...