Apple Watch Ultra 3 cellular compatibility between regions
can i use a watch ultra 3 bought in Thailands apple store in germany?
i. e. does it have the same cellular band width?
Generally, Apple recommends to use the Apple Watch in the country of purchase, due to different hardware which might be built in to support your local carrier.
And, there is also the fact, that Apple may restrict the warranty to the country of purchase. In case of a warranty claim, you might have to ship it back to the country of purchase.
...
To cover the LTE and UMTS bands used around the world, there are two separate cellular models for Apple Watch. 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 cellular service worldwide. To see which carriers in your country or region offer cellular service for Apple Watch, refer to the chart below.3
...
copied from: Apple Watch - Carriers - Apple
And you will always have the country specific watchOS installed, and some features might not be available as they are in other countries.
watchOS 26 - Feature Availability - Apple