Sony Ericsson Xperia Arc S on AT&T 3G

[gerodotfour]

I've got a Sony Ericsson Xperia Arc S (American version, the LT18a) and can't seem to get anything more than the EDGE (2G) network connectivity on AT&T. I updated my SIM card the other day just in case somehow that was the issue, and have been trying many internet searches to no avail. I had to manually input the Access Point Name (APN) information to get any data network connectivity at all, and here is what I've got:

Name: AT&T
APN: wap.cingular
Proxy: Not set
Port: Not set
Username: WAP@CINGULARGPRS.COM
Password: xxxxxxx
Server: Not Set
MMSC: http://mmsc.cingular.com
MMS Proxy: wireless.cingular.com
MMS Port: 80
MCC: 310
MNC: 410
Authentication type: Not set
APN Type: Not set
APN protocol: IPv4 but text is grayed out and can't edit without specifying APN Type

Anybody out there know how to fix this issue? The phone is advertised as 3G capable but I haven't figured out how to do it yet. No support from AT&T since it's unlocked. Appreciate any assistance provided.