Is it mandatory to connect Apple Watch cellular to a cellular provider for it to work?

Hi

No, it is not mandatory.

Apple Watch Series 3 (GPS + Cellular) can be paired, set up and used for other features whilst skipping the steps to activate cellular. It will then function like a GPS-only model.

The built-in cellular feature can be enabled later, if and when desired, subject to having a compatible service plan on your iPhone that specifically includes support for the Apple Watch, including the carrier activating the watch on the plan.

• Apple Watch - Carriers - Apple
• Set up and use cellular on Apple Watch Series 3 (GPS + Cellular) - Apple Support

In the meantime, it can be used in conjunction an iPhone 6 or newer model (running the latest version of iOS) with a service plan from any carrier.

The functions that are available on your Apple Watch when it is away from your iPhone depend on various factors including:

• Your Apple Watch model - Series 3 (GPS) vs Series 3 (GPS + Cellular):

  Apple Watch - Compare - Apple

• The features of your carrier service plan:
  • Apple Watch - Carriers - Apple
  • Make a call with Wi-Fi Calling - Apple Support
• The type of connectivity that is available to your Apple Watch at the time:

  About Bluetooth, Wi-Fi, and cellular on your Apple Watch - Apple Support

Features and requirements that apply under various circumstances are detailed here, including within the footnotes:

How to use your Apple Watch without your iPhone nearby - Apple Support

You're welcome! 🙂