Apple Watch Series 3 (GPS + Cellular) models may only work to provide cellular services in the country where they were originally purchased, according to a new report by MacRumors. An Apple support representative told a MacRumors reader that an “Apple Watch purchased in the U.S. online store will only work with the four carriers in the United States.” While no additional details were provided, it seems that the problem would most likely be due to a limited number of LTE bands supported by the new cellular-capable Apple Watch model.

Unlike the iPhone, engineering constraints in the much smaller Apple Watch likely limit the number of frequencies that can be effectively supported, and with carriers in different countries supporting different LTE frequency bands, it seems likely that Apple would optimize each Apple Watch Series 3 for cellular services in the country where it is being sold.
That said, the response from the Apple support rep only specified that Apple Watch sold in the U.S. would not be compatible outside of the U.S. It’s less clear whether this restriction will exist with Apple Watch Series 3 models sold in other countries, such as within the EU, where common LTE bands are used between countries.
For example, Apple’s Apple Watch Carriers page seems to suggest that there are only three country-specific models of Apple Watch — one for use in the U.S., Canada, and Puerto Rico, one for use in Australia, Japan, and the EU, and one for use in China. It therefore seems likely that, in the absence of any artificial restrictions such as carrier locking, each of these models could be activated in any of the countries listed for that model, regardless of where they were originally purchased.
Apple has also already made it clear in the fine print that Series 3 users won’t be able to use the Apple Watch cellular services when roaming in a foreign country, although this limitation is related to incompatibility of eSIM and “NumberShare” technology with roaming services.