In case anyone else comes along... I have found these settings work for AT&T Lumia 635:
Leave the rest blank
(I do see an Authentication type set to 'PAP' but I can't change it)
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.
Thank you for this post!! I've had several Chat and Phone Support calls in for my Nokia Lumia 635 on Straight Talk and at no time have they gotten the settings to match what you had posted here. Amazing, that once I plugged in these settings, within 15 minutes, I can now send Picture TXT to all my phones, and receive them as well.
I don't know if any of these settings are 'region specific' but if it matters, I live in Seattle, WA area. There are 2 pieces of the puzzle that I was never given exactly as above:
1. The WAP gateway settings. They always told me to leave them blank.
2. the IP Type: IPv4 - Initially it was set and not change-able from IPv4v6 and I needed to 'delete' both the APN and MMS settings, then create new ones with the above listed settings, and it works great now.
Thanks for sharing. Hoping Straight Talk Support will post this more visibly as the fix, as so far I've gotten everything from "It's a Windows Phone, and we don't have the settings, so there is no guarantee" to "I am the supervisor, and the settings we've given you are correct to our knowledge, please allow 24 hours for the proper information to be downloaded to your phone, and call back if it did not fix the issue" which it never has.
Using Forums will save costs for Straight Talk as often times people can fix their own issue with a simple "Internets" search and be on their way.
One more thing I've heard on some of the posts here and on the Interwebs: No, you don't need to 'unlock' the Nokia Lumia 635 Go-Phone to use it with Straight Talk. It will work, if you follow the instructions scattered around on how to set it up, no unlocking required.
Thanks for sharing and listening. Hoping this post gets saved for all the other future users who run into this problem, and struggle to find a fix.