Bell does support LTE connectivity of the Apple Watch smartwatch. On your iPhone, you will need to update your Carrier Bundle to version 29.1 or newer. To do so, follow these steps:
Make sure your iPhone is connected to a Wi-Fi or cellular network.
Tap Settings > General > About. If an update is available, you'll see an option to update your carrier settings.
To see the version of carrier settings on your iPhone, tap Settings > General > About and look next to Carrier.
Note: if you’ve recently inserted a new SIM card into your iPhone, you may need to update the carrier settings.
For more information on Apple’s carrier settings, visit: https://support.apple.com/en-ca/HT201270