With the LTE-enabled Apple Watch Series 3 only available in a handful of countries at the current time, it might be tempting to purchase one in another country, but it won't work. Series 3 models appear to be limited to connectivity in their original 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.
懶人包：
哪個國家買的LTE版 watch因為硬體限制只能適用於該國的電信商（出國漫遊也無法使用）；所以想先去外國買LTE版等台灣支援的可以收手了
來源：http://bit.ly/2xFdxGr