I was having the same problems with my Nokia Lumia 635. Straight talk gave me different settings of:
APN: att.mvno
WAP: proxy.mvno.tracfone.com
WAP Port: 80
MMSC: http://mmsc.cingular.com
MMS Port: 80
I tried the initial settings which didn't work and they gave me these new settings to try. It actually did work for 1 whole day and now my phone is back to not receiving or sending MMS messages. Is there a fix to this or are these phones not compatible with Straight Talk?