Hi, Creating a new thread, regarding the same issue, probably won't get you help any sooner. So let me get this right, you're in an AT&T forum for Note 3, asking for help with a Note 3 that's on a different carrier? Its very doubtful that if your own carrier can't help you, that anyone here could. But, you never know. Here's what you can try...
Download an app called Textra (text messaging app), see if it works initially; if it doesn't click on the settings within Textra and scroll down to the MMS section. You'll see a spot that says MMS, look for "Go Here if MMS Isn't Working!" Then look at the bottom where it says, "Manually Configure APN." See if you can get it going by entering your phone's setting in there, if its not already there.
Hope this help!