Can't really help with your question but wanted to say that I don't see the point in having a data plan on a Watch.
I had a data plan on my old S4 watch but found that 99.99% of the time, I always had my phone with me when also wearing the watch so the plan was a little pointless.
Now that I have a S7 watch (with no plan), I don't miss not having a data plan at all - the only thing it does is allows you take phone calls without your phone anyway, and realistically, who would even do that?!!
I use my watch to track running as well and even without a data plan, it can still track the route of your run, even when you do not carry your phone on your run (which I don't).