Okay ... I took the chance on this method (including hard resetting my phone) and it worked!!
I have am a brand new customer of T-Mobile and have an Verizon Icon. Two of my kids also own Icon's. I have been trying everything, including this method (without hard resetting the phone) and nothing worked. I came back here tonight and said, okay this is the last resort. I hard reset my Icon and followed the instructions and I now have LTE and MMS on T-Mobile using my Icon.
I can't thank the OP enough for posting this. I was actually thinking the 3 of us were going to have to get new phones, and as many know, there are not any Windows phones better than the Icon.
Thanks!!