According to a statement the company gave to The Verge, they’re aware of the issue and are investigating a fix that will be released as part of a future watchOS 4 update.
We have discovered that when Apple Watch Series 3 joins unauthenticated Wi-Fi networks without connectivity, it may at times prevent the watch from using cellular. We are investigating a fix for a future software release.
Kudos to Apple for acknowledging the issue just after pre-orders. The Verge editor Lauren Goode wrote in her Series 3 review:
On more than one occasion, I detached myself from the phone, traveled blocks away from my home or office, and watched the Watch struggle to connect to LTE. It would appear to pick up a single bar of some random Wi-Fi signal, and hang on that, rather than switching to LTE.
While many reviews point out that they had issues with calling in general from the Apple Watch Series 3 with LTE, others don’t seem to have any issues at all. Some believe that the battery life is going to be an issue for a lot of future owners, and even Apple confirmed the wearable has only one hour of talk time, but talking on the phone, let alone from a smartwatch, for an hour might not be a big deal for most.