In case anyone else comes along... I have found these settings work for AT&T Lumia 635:
Internet APN:
APN: att.mvno
Leave the rest blank
(I do see an Authentication type set to 'PAP' but I can't change it)
MMS APN:
APN: att.mvno
WAP gateway (URL): proxy.mvno.tracfone.com
WAP gateway port: 80
MMSC (URL): http://mmsc.cingular.com/
MMSC port: 80
Maximum MMS size: blank
IP type: IPv4
These worked out great for me. If this did or didn't work for you, please reply. I would be interested to know.