So I have been struggling with no MMS on my Sprint Ativ S Neo since shortly after upgrading to 8.1. I had been through three levels of Sprint tech support and they said it should be working. So, this morning I decided to do a hard reset. I did not restore from backup, but started fresh and everything is working fine again. Not sure what happened exactly, but for anyone who is only dealing with MMS which isn't working, this worked for me. Thought I'd pass it along just in case it could help someone else out.