iPhone 6 straight talk can't send mms messages

[iMore Question]

I Have an Apple iPhone 6 and am using a straight talk no contract plan. I set it up on Sunday, and today, Tuesday, I started getting an error message saying iPhone is not activated and to contact my carrier. Everything still works, except for mms messages. I can no longer send mms messages. I still receive messages, but cannot send messages. I can still make and receive phone calls, and can still use the Internet. I just can't send mms messages! I also can only send iMessages if connected to wifi. I called my carrier and talked to them for over an hour. They did every trouble shooting optio, and it still doesn't wor. They told me to contact Apple support, however my coverage through Apple is expire. Any suggestions