This is a problem that has been occuring alot. I too, have the same problem and so do many people who own the one x and are connected to O2. The only difference is that they have it fixed. http://community.o2.co.uk/t5/HTC/HTC-One-X-Manual-APN-Settings-Solution-Found/td-p/225729 

Does anyone here know how to adapt this solution so that it can work for att users?