It's been 6 months since I switch back to StraightTalk from H2OWireless with AT&T Network. My Nexus5 hasn't had way to successfully send MMS. I was able to send MMS before switching over to H2OWireless.
The problem is that the original StraightTalk APK setting on my Nexus5 no long works. So I Google it a lot. Try many thing and nothing really works. BTW, I am using Google's Hangout App as my default SMS app. I also use Google Voice on Hangout as well. My temporary solution was to use Google voice on Hangout to send and receive MMS.
Now after so many tries, I finally get MMS working on Nexus5 with StraightTalk on AT&T network.
Here is the screenshot of the APN Setting. I hope someone having so much trouble find this helpful.
The only few difference than other people's setting are the APN Type, Authentication Type and APN Protocol.