Thank you and all the wonderful people who have contributed to this discussion. I have the same problem: I am using ST on an unlocked, ATT compatible phone, the HTC One (old version). I have the micro sim card and had been sending/receiving MMS messages just fine until the update from Android 4.4.1 to 4.4.2. I, too, have spent hours and hours on the phone with HTC and ST and more hours reading all I can about this issue on various forums. I have tried various combinations of APN settings and finally thought I had found the answer when I read your post. But, alas, your suggestions did not work for me.
I looked at the post on XDA-developers about 'rooting' my phone, but I know next to little about this technology and although I am good at following directions, if those directions are not written explicitly 'enough' (as you point out in your post concerning shutting down and restarting your phone between updates to the APN settings), I may be really up the creek without a phone, if I screw up. I bought it unlocked online and have no insurance. I would surely be grateful to any of you who has another idea! In the meantime, blessings to all of you.