The Series 9 and the Ultra 2 are the first Apple Watches that have had the same serial numbers.
Most of us volunteers are waiting to see if users are able to use a U.S. purchased Apple Watch in the EU, UK, or other country. Or the opposite, an Apple Watch series 9 or Ultra 2 purchased across the pond being used in the U.S., Canada, Mexico or Puerto Rico.
As we are uses just like you, we do not have any inside information. We only have what the
https://www.apple.com/watch/cellular
website says, and then we can speculate.
But until we get enough confirmation that the same Apple Watch works anywhere in the world, we will not know for sure.