Hi
The built-in cellular capability on Apple Watch Series 3 (GPS + Cellular) is only used when your Apple Watch is unable to connect either via Bluetooth or, as a fallback to a trusted Wi-Fi network.
It may help to check that your Apple Watch Series 3 is connecting to your home Wi-Fi network successfully when it cannot connect to your iPhone via Bluetooth:
Firstly, ensure that your Apple Watch and iPhone are connected via Bluetooth and also that your iPhone is connected to your Wi-Fi network:
On your iPhone, go to: Settings > Wi-Fi:
- Tap on the "i" / info icon to the right of your Wi-Fi network name > tap on Forget This Network.
- With your Apple Watch still connected via Bluetooth, reconnect your iPhone to your Wi-Fi network, being sure to check that you are connecting to a 2.4 GHz network.
- Pause for a few moments to allow the Wi-Fi information to be shared to your Apple Watch.
On your iPhone, enable Airplane Mode:
Check whether your Apple Watch has successfully connected to the Wi-Fi network by checking for the Wi-Fi icon at the upper-left of Control Centre (allow a short while for it to connect after losing the connection with your iPhone):
When finished, turn off Airplane Mode on your iPhone.
If you have not yet done so and if the feature is supported on your plan, it may help to check that Wi-Fi Calling is enabled on your devices:
- On your iPhone, go to: Settings > Phone > Wi-Fi Calling.
- On your iPhone, in the Watch app, go to: My Watch > Phone.
Make a call with Wi-Fi Calling - Apple Support
More information:
About Bluetooth, Wi-Fi, and cellular on your Apple Watch - Apple Support