https://www.theverge.com/circuitbreaker/2017/10/4/16424590/apple-watch-series-3-update-connectivity-issue

Apple's new Apple Watch Series 3 had some serious LTE connectivity issues, and the company's now trying to address them with an OS update. The Verge's Lauren Goode wrote in her review of the wearable:

"It’s reasonable to expect it might take a minute for the Apple Watch to 'find' LTE after you’ve walked away from your phone. It’s not reasonable for this to take many minutes or not work at all. This almost makes me wish there was a way to actively turn off Wi-Fi on the Watch, so it would just default to LTE."