Previewed at WWDC, launching in the fall.
LTE in Apple Watch Series 3 Models Only Works in Country of Purchase
An Apple support representative who spoke to MacRumors reader Thomas said that an Apple Watch purchased in the U.S. online store will only work with the four carriers in the United States, perhaps due to hardware limitations.
"If you purchased one (an Apple Watch S3 GPS + Cellular) through the US Apple Online Store, it'll only work for the 4 US carriers. It wouldn't work internationally unfortunately. I'm not entirely sure what type of error it would show if you took a US Watch to Germany, but it wouldn't be compatible with Telekom".Additionally, Apple Watch Series 3 models do not support roaming outside of a provider's network coverage area, according to Apple's fine print, so roaming when traveling to another country is not supported.
Unlike iPhones, Apple Watch Series 3 models also support a limited number of LTE bands, another factor that contributes to no support between countries. In the U.S., Canada, and Puerto Rico, for example, only bands 2, 4, 5, 12, 13, 17, 18, 19, 25, 26, and 41 are supported.
LTE Apple Watch models are currently only available in Australia, Canada, China, France, Germany, Japan, Puerto Rico, Switzerland, the UK, and the United States. Apple says availability will expand to other countries next year.