Today, Apple released watchOS 4.0.1, an update that is designed to fix a Wi-Fi issue that some Apple Watch Series 3 models with LTE cellular capabilities experienced. The issue, which had caused some early Series 3 reviewers to pan the device when they ran into the problem, caused the Watch to connect to an inaccessible but familiar Wi-Fi network instead of to an LTE network.
As noted in the update notes seen above, "watchOS 4.0.2 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."
To install the update, go to the Watch app on your iPhone, go to General > Software Update. Place your Apple Watch Series 3 on its charger (it must have at least 50 percent of a charge to even begin downloading the update), and make sure it is connected to Wi-Fi.