The guest asked his question five months ago, but I have a similar problem, so my workaround may still be relevant to some users of the HTC 8XT. I receive notifications of MMS messages, but am unable to download them when my phone is set to use both CDMA and LTE, as I usually have it set. When I receive a message via MMS, if I go to Settings > cellular+SIM > SIM settings > Mode selection, and change the setting from "LTE and CDMA" to "CDMA," I can download the MMS message. Afterward, I change the setting back to "LTE and CDMA."
Obviously, this is just a workaround, and it's inconvenient. However, if one doesn't receive many MMS messages, as I do not, then this workaround may be just good enough. Since finding the workaround, I haven't taken time to find a full solution.
I hope this helps someone.