Hello fahama,
Apple Watch only supports one cellular line at a time. If you currently have the eSIM set up with a carrier, you'll need to remove that cellular plan to be able to setup the eSIM with another carrier. Your paired iPhone also needs to be on the same wireless carrier that you'll be setting up the watch on. So if you're switching to a different wireless carrier, you'll want to do it on your iPhone first, and then you can change the carrier on the watch.
You can find more info on using cellular service on Apple Watch here: Set up cellular on Apple Watch
Cheers!