A-te-lee-ay. So just the French pronunciation, really.
When I was a kid I grew up saying it in a lot of ways, main one I remember is a-tee-lee-ur I think.. pretty sure my first hint at the actual way to say it was when I first listened to At Atelier by Fake Type lol. But tbh it's entirely possible and even likely that years before that song I looked it up as a kid and then just forgot again.