Using a non MTS or grey market device for MMS can sometimes be tricky due to the fact that the APN settings required for it have to be manually entered and sometimes these devices wont fully function on our network at all. A couple suggestions you can look into is first to ensure hangouts is not enabled or setup to manage your MMS. This program will certainly cause difficulties sending messages. The next thing to look into would be to verify and even re-enter your APN settings. These settings can be found HERE. I hope one of these options gets it working for you, if they don't you can certainly speak to our technical support group however our support of grey market devices is quite limited as we can't be certain how these types of devices will work with our network.