HTC Vivid - multiple issues

We got the Vivid a few months ago.  We still have yet to be able to connect via 4G, only by wifi.  We have never been able to receive MMS messages, and just now, for the 3rd time, text messages have stopped.  They freeze up and say "loading".  I have been on chat with AT&T and HTC.  Both have had us reset the phone, and that has worked for texting, but nothing else.  The HTC rep said she had a phone she had to get AT&T to put 4 sim cards in, and that maybe ours is not provisioned correctly.  I tried another SIM card in it, and same issues.  I am at my wits end.  I have not even bothered spending hours in our local AT&T store, because I have read on so many forums that noone has a clue what to do.  Does anyone have any suggestions?  Is it AT&T's problem, is it HTC?  What is the deal and how can I fix this?  Ughhh!