Apple has released a new software update for the Apple Watch that fixes the LTE connectivity issue. The watchOS 4.0.1. update fixes the WiFi issue some Series 3 with LTE models experienced.
Few Apple Watch Series 3 devices were struggling to connect to LTE and often linked to an unknown WiFi network instead. It would try to pick up a single bar of some random Wi-Fi signal and hang on that instead of switching to LTE.
Apple writes in the Apple page,
watchOS 4.0.1 fixes issues that in rare cases were causing Apple Watch to join unauthenticated (captive) Wi-Fi networks, such as those found in public places like coffee shops and hotels, which direct the user to a webpage before the network can be accessed.
watchOS 4.0.1 can be downloaded through the dedicated Apple Watch app on the iPhone by going to General –> Software Update. Apple is also currently in the process of beta testing watchOS 4.1, which will likely be released later this year to the public.