Apple iPhone 3GS

I unlocked the phone in april and have the latest os and use an at&t sim that has worked for four months and use the straight talk service from walmart. I could use wifi no problem but outside of that it has a message of unable to connect to server. I went to network under settings and it shows vpn not apn. Does that mean that at&t sent an update that locked the phone again? I went to unlockit.co.nz and like the first time thought after I deleted the old info and added the new apn info it would work but no luck. Any thoughts or suggestions? I am a non techie so if you can explain in a simple way that would be great! Thank you!