How to change carrier in Qatar

The salesmen said it is unlocked

The salesman is wrong or you misunderstood. All iphones sold in the US are carrier locked to AT&T with no official way to unlock them.

What should i do?

Sell the phone & buy one you can use.

You misunderstood the sales person. All US phones are locked to ATT and won't work with your SIM in Qatar.

You will need to either sell the phone you have or get it to someone in the US who can return it for a refund.

Same here, 2 weeks ago I bought an iPhone in NYC. Apple had this 'new' offering 'buy as a gift'. So did buy this iPhone 'as a gift' for 599$/no contract. The salesman @ apple didn't mentioned at all that the phone is still locked at AT&T and that it would not work with another carrier, i.e. in Europe. The salesman knew that we were not from the US. So my gift for the moment is worthless here in Europe.
I'll try to find a solution at our local Apple store... let's see what they will offer me.

The salesman may have thought you were buying a gift for someone in the US. It's not their responsibility to ask you who the gift is for.

The Apple Store in Switzerland is not going to be able to do anything for you. Only ATT can unlock the phone and they do not offer unlocking at this time.

Hi Tamara, you are right, only ATT can unlock devices. Actually the store manager gave me a very good advice. Let's see if this is going to work for me too. I will keep you updated - but this is going to take same time as I will have to send this iPhone back to a colleague in the US who will contact ATT. In the meantime a have a new one - had to, as I need the gift for next week. Anyway... 😉