https://www.theregister.co.uk/2017/09/20/reception_woes_apple_watch/

The new LTE-enabled Apple Watch 3 appears to suffer from a bug that can keep the touch-screen wearable from connecting to cellular data networks.

Multiple reviews of the still-unreleased device have found that when wearing it without its paired iPhone, the Apple Watch constantly tries to connect to nearby Wi-Fi networks, rather than activate the LTE data connection.