If you set up multiple cellular plans using an iPhone with Dual SIM, you can add multiple lines to your Apple Watch with cellular, then choose the one your watch uses when it connects to cellular networks.
Note: Each iPhone cellular plan must be provided by a supported carrier and must support Apple Watch with cellular.
You can add one plan when you set up your watch for the first time. You can set up a second plan later in the Apple Watch app by following these steps:
Go to the Apple Watch app on your iPhone.
Tap My Watch, then tap Cellular.
Tap Set Up Cellular or Add a New Plan, then follow the steps to choose the plan you want to add to your Apple Watch.
You can add multiple lines to your Apple Watch, but your Apple Watch can connect to only one line at a time.
Go to the Settings app iPhone User Guide.