Hi everyone. I got a new iPad 4G with an AT&T Sim card. I did the initial setup and I activated everything getting online with the AT&T service from the SIM card. I have been using WIFI in my house for a while, and now seems like the AT&T SIM card doesn't work... or probably, I just don't know how to use it. I want to connect via the SIM card. How do I do it? Do I need to buy another SIM card? Or do I just activate this one? I don't want to sign up for a long-term plan, just something prepaid if I can.