phone wont connect to data and wont recieve mms messages

Hi, I have a unlocked Nokia Lumia 521 which has were perfectly since December. All of a sudden it just stopped recieveing data and messages. I talked to technical support manager and she told me to go up to the store and have them enter the access point number and it should work just fine. So I go to store and they have no clue what an APN is. I ask for the manager she has no clue as well. The manager says,well you can fix this problem by just upgrading your phone today sir. Why? So you can make commission off of me,instead of trying to fix the problem? I have never dealt with such horrible customer service in my life. I've spent hours on the phone and not one person can help me. They just want their money. Oh just upgrade sir were to lazy to help. Get the next program and end up paying 500 for a iPhone. No thanks att. Someone has to know how to fix this problem. Please help