Using Apple Watch Cellular in Germany

No.

To cover the LTE and UMTS bands used around the world, there are two separate cellular models for Apple Watch Series 8, Apple Watch SE and Apple Watch Ultra. They have been optimised 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 network providers in your country or region offer cellular service for Apple Watch, refer to the chart below. 

Apple Watch - Carriers - Apple

You can enjoy the Ultra as a GPS-only watch with almost all the regular features, you just need to keep your iPhone with you. That is something that most people do anyway! Turn off the cellular in the Watch.

Main thing is No Apple Support and No Warranty for Watches bought outside your home country

xzeroxx wrote:

Is it possible to buy the Apple Watch in Singapore and use it in Germany with my German phone?

Maybe.

Using this link:

https://www.apple.com/watch/cellular

check the models used in Singapore vs those in Germany.

But keep in mind the warranty is country specific, so you would have to return to Singapore if you wanted warranty service.

Also some countries limit features on iPhones sold in their country, so check to make sure the features you want are available on the Singapore models

https://www.apple.com/watchos/feature-availability

No, so don't buy the watch in the US with the hope of connecting to cellular bands in Germany. All Apple Watches sold in the US are specifically made to support cellular bands in the US, Canada, Mexico and Puerto Rico. If you live and will use the watch in Germany, buy the watch in Germany. You'd also have NO warranty on a watch purchased in the US if you need warranty support in Germany.

Is it possible to buy the Apple Watch in Singapore and use it in Germany with my German phone?