not able to send MMS on i phone 4s on tmobile network

so i just switched my data plan from blackberry to i phone thinking it would give me 4G capability. sadly, they don't offer it here yet 😝 however, now i am unable to send MMS to anyone!

i had this problem initially when i got the phone, and i remember someone gave me some information regarding changing some settings, but it isn't in my history. i already reset the phone, turned off and on messaging, and nothing. any advice?