US Apple Watch 7 GPS + Cellular functioning in Hungary

Hi Apple Team,

I got an Ape Watch 7 GPS + Cellular as a present, originally it was bought in the US.

I live in Hungary and I’d like to use the cellular functions of the watch, but when I’m trying to add the service it doesn’t work out.

What do you suggest, how could I make it work?

Thanks a lot for your input in advance!

Tamás