Why won't my data work on my iPhone 3GS?

I got my iPhone 3GS from a friend and he was on AT&T so my phones a carrier for AT&T and I bought a straight talk phone from Walmart and activated it and put the sim in the iPhone everything works but the data and mms I've tried this website http://www.unlockit.co.nz/ it didn't work ive used the APN that you can get on the straight talk website and it didn't work I looked everywhere and I can't find why my straight talk data won't work for my iPhone 3GS please help!

Did you get the Straight Talk BYOD SIM card or did you try to use a SIM card from a GSM Straight Talk phone?

If you did not get the BYOD SIM, that is why the data is not working, as the SIM cards for the basic Straight Talk phones are linked to the phones they are sold with and will not provide mobile data to other phones, and quite frankly, I am surprised that the talking and text work on the iPhone 3GS in question if that is indeed the case.