APN Settings

I have a factory-unlocked iPhone 4G. I use a prepay service in Canada with unlimited Internet usage. The CSP is 7-11 SpeakOut, a MVNO that operates on the Rogers network. Voice and data services work fine.

I am currently in USA. I have subscribed to a pre-pay service with data. I've replaced Canadian SIM with American SIM. American CSP is also 7-11 SpeakOut, however, in USA, SpeakOut works on the AT&T network. Voice service is working fine. Data is not. When attempting to connect to Internet, error messages advise that data services have not been activated (CSP does not offer data service) or that server cannot be reached. I have confirmed with SpeakOut USA that data service is active on my prepay service. The service bar shows the presence of 3G or (occasionally E for Edge service). iPhone shows SpeakOut as the network but it shows AT&T 9.0 as the carrier. SpeakOut customer service provided the following APN settings:
APN: wap.cingular
User Name: WAP@cingularGPRS.COM
Password: CINGULAR1
Proxy Server: left blank
Port: left blank

These settings do not result in any data connection. I get the same error messages as before.
One point of clarification, a company called Ztar is actually the CSP. 7-11 is actually a sales distribution channel for Ztar's services in Canada and USA.

I suspect that the APN settings that I was given are incorrect and need to be changed. But to what???