When I switched to ST from AT&T after they didn't like my have a Galaxy S4 (GT-I9500) I was giving the following settings for my phone:
Name: Straight Talk
APN: att.mvno
Proxy: proxy.mvno.tracfone.com
Port: 80
MMSC:
http://mmsc.cingular.com
MMS proxy: 66.209.11.33
MMS port: 80
Recently though I upgraded my wife from her old Motorola Defy+ to a Galaxy S3 (GT-I9300) and changed her ST SIM from T-mobile to AT&T network and got a new APN for her from during setup.
New (current as of this post on ST support for BYOD page) APN:
Name: straight talk
APN: tfdata
MMSC:
http://mms-tf.com
MMS proxy: mms3.tracfone.com
MMS port: 80
I've seen random reports online of people losing access to data after their renew data until they enter the "current" APN settings.