In general Apple strongly recommends against purchasing mobile devices for use in another country.
Apple Watch - Carriers - Apple - https://www.apple.com/watch/cellular/ "To cover the LTE and UMTS bands used around the world, there are two separate models for Apple Watch Series 5, and three for Apple Watch Series 3.2 They have been optimized for use in the country of purchase, and not all models work in all countries or regions. There is no single model that supports roaming worldwide. To see which carriers in your country or region offer cellular service for Apple Watch, refer to the chart
LTE Find the right iPhone for the right country or region. - http://www.apple.com/iphone/LTE/
Warranty restriction! - http://www.apple.com/legal/warranty/products/ios-warranty-document-us.html - "IMPORTANT RESTRICTION FOR iPHONE, iPAD AND APPLE TV SERVICE. Apple may restrict warranty service for iPhone, iPad and Apple TV to the country where Apple or its Authorized Distributors originally sold the device."