Iphone 5 says it has 3g but will not connect to the internet?

I have a Straight talk plan, my phone says I have 3g, but will not connect to anything on the internet unless I'm connected to Wi-Fi. I have tried reseting the network connection, restarting the phone, and restoring the phone back to its old settings, nothing has worked. Any suggestions on how to fix this?

Connect your iphone to a Wi-Fi connection. On your iphone safari internet search for iphone apn by unlockzit. Set up the apn and download the app for straight talk.