I had the same experience. I was very excited to try a Doner in Istanbul, because one of my favorite restaurants in Seattle was a Doner place (The Berliner). They didn't seem to know what I meant. I got something they insisted was a Doner, but not really the same sort of thing.

Not sure if the Seattle place is really Americanized or I need to save up and try one in Germany.

Plenty of great Turkish food btw, but obviously Doner's are not really a local thing.