I agree with kawgirlval69. I am on stock GB rom with franco.kernel v10 and i have no problem sending mms wifi on or off. Your apn has to match your plan AND your sim card. It also has to match the the data plan att has coded in to your account. You can't use a LTE apn with a non LTE sim card. The link 2 or 3 posts above is a great thread from Jade Eyed Wolf about setting up the correct apn. Read it. It works.
Sent from my Beautiful, White GT-N7000 using xda premium
Samsung Galaxy Note GT N7000 White & Rooted with 32GB Cl.10 External SDHC (Currently)
Dell Streak 5 running Simple Streak 1.2 PerfUpdate + OC 1.190GHz (Formerly)