This ultimately defeats the point of having an LTE option in the first place, plus joining unknown WiFi networks is also something of a security issue. Apple had caught wind of the problem ahead of the device’s release, and while the Apple Watch Series 3 has since been released, the good news is that Apple has released an update to watchOS that fixes the LTE bug.
According to the watchOS 4.0.1 changelog, it reads, “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 web page before the network can be accessed.”
If you are one of those who are affected by this bug, then you’ll want to update to watchOS 4.0.1 as soon as possible as the update should already be live.