Lumia 900 unlocked (from ATT) cannot send/receive MMS

Hello all.

I just switched from ATT to Tmobile. My phone is an ATT Lumia 900, unlocked.

Phone and internet works fine, SMS works fine.

however I

- cannot send MMS (it says failed to send message, try again)

- cannot receive MMS (I do receive the text of the message, but not the picture).

Is there any network setting I need to change, or APN I need to add for this phone so that I am able to send/receive MMS?

Thank you.