The Cellular + GPS Apple Watch shares the iPhone's phone number, and the carriers that support the Apple Watch charge a monthly fee. In the U.S. the fee is $10 per month. Here is a list of carriers that support the Apple Watch:
https://www.apple.com/watch/cellular
You cannot use a separate cellular plan.
The closest you can come is to use a Family plan, where there is a parent's iPhone managing the Apple Watch, but even then the carrier has to support the Apple Watch via Family Plan, and the features on the dependent child's Apple Watch are less than the parent's Apple Watch, because the dependent child is not allowed full access to the parent's iPhone.