I have a Samsung Galaxy Centura phone running Android 4.0.4. purchased through StraightTalk. I am unable to send MMS messages. I have found plenty of references to set up APN settings to I can achieve this, yet for some reason, my phones do not even show APN settings. When I go to "wireless & networks", then to "mobile networks", I only have a "data enabled" check box. How do I get this to work?