Understanding LTE on Apple Watch Series 3

Hi

It is not necessary to close apps on Apple Watch.

In order to use the built-in cellular feature on Apple Watch Series 3 (GPS + Cellular), you need a service plan for your iPhone that specifically includes support for watch, including the carrier activating the device on the plan:

Apple Watch - Carriers - Apple

Apple Watch Series 3 (GPS + Cellular) connects using the most power-efficient available method. This includes connecting to Wi-Fi as a fallback whenever a Bluetooth connection with your iPhone is not available and only connecting to the cellular network if neither a Bluetooth nor a Wi-Fi connection is available:

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

Any data consumed by the Apple Watch is consumed either:

• Via the paired iPhone (including Wi-Fi and cellular data).
• Via a direct connection between the watch and a compatible Wi-Fi network (when the watch cannot connect to the paired iPhone via Bluetooth).
• For Apple Watch Series 3 (GPS + Cellular) models, via the built-in cellular feature, only under the circumstances described above.

If you have an Apple Watch Series 3 (GPS + Cellular) model that has been added to your carrier plan, then the built-in cellular feature can be turned off, if desired, via Control Centre:

Use the Control Center on your Apple Watch - Apple Support

More information:

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

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

Use Apple Watch with a cellular network—Apple Watch Series 3 (GPS + Cellular) only