
Apple Watch uses both Wi-Fi and Bluetooth to transfer data to the iPhone. It defaults to a Bluetooth connection whenever possible because it conserves power, but if your iPhone is not in Bluetooth range, Apple Watch will switch over to compatible Wi-Fi if it is available.
For Wi-Fi to work on Apple Watch, it needs to be 802.11b/g/n on a 2.4GHz band. It will not connect to the faster speed 5GHz Wi-Fi, nor will it connect to public networks requiring logins, subscriptions, or profiles. It will also not work if your paired iPhone has never connected to the local Wi-Fi network before. This is because Apple Watch cannot configure new Wi-Fi networks on its own. It can connect to networks that you have set up on the paired phone.

When your paired iPhone is out of range, and it is not connected to the same Wi-Fi network, there are a number of features that are still available:
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Article Link: What You Can Do With Apple Watch When Your Paired iPhone is Out of Range