Thanks.
By default, the Apple Watch communicates with the iPhone via Bluetooth. If the iPhone is not in the Bluetooth range of the Apple Watch, or the iPhone has turned off the Bluetooth connection, the Apple Watch will connect directly to a nearby known wireless network through the built-in WLAN chip. The Apple Watch does not provide a WiFi setup interface directly, so it is connected by reading the data of the iPhone, and can not directly connect to the new network. When there is no WiFi network available nearby, the LTE version of the third-generation Apple Watch will automatically connect to the cellular network (when supported).
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At present, after the second and third generation Apple Watch upgrades watchOS 4, the control center will have a WiFi switch, which is not provided by the first generation, and the third generation LTE version will also have a cellular switch.