No Internet On Lg Gd570 D'lite T-mobile Phone Unlocked For At&t?

My daughter had her heart set on the LG D'lite from the first time she saw it. Even though it was a t-mobile phone and we have at&t, a vendor at a cell phone kiosk said he could fully unlock it to work on the at&t network. She worked really hard to earn the money to buy it at full price, then paid $60 more to have it unlocked. When she picked up her phone from the kiosk, she was able to make and receive calls and messages, but was unable to connect to the internet. The guy at the kiosk said to check with at&t, which she did, but they were unable to get the internet connected. I am very technically challenged, but did some research and found that:  1) the phone's "firmware" may be hard-coded to t-mobile settings, 2) it is not known if the settings can be changed.  I know t-mobile's policy is that they will give up the unlocking code after 90 days of service. My questions are:  1) is there any way to find out if the settings can be adjusted so she can use the internet, or should i just sign up for 90 days of service with t-mobile? (and is it even posssible to sign up for only 90 days of service?)  2) if the guy at the kiosk said he could "fully unlock" the phone for $60, does this internet problem constitute that he did not perform a "full unlock?" (i don't even know - - is that a reasonable price for just unlocking the phone portion? It seems a little pricey, but then again, I have never "unlocked" a phone before.) At this point, I would really appreciate any information. My daughter is heart-broken, this was the first big-ticket item she bought with money that she worked really hard to earn. And I feel like a crappy mother for telling her that if she wanted that phone, she would have to earn her own money to buy it. (It was supposed to be a responsibility-building lesson!)