Just curious OP,
Did you try resetting the APN under "mobile data" settings?
I recently switched carriers on an unlocked vanilla HTC sense ROM, and even though the APN *claimed* to be on "auto", I still had to enter the APN info by hand.
Once I did my 4g was up and running.
This place is a good example, but you'll have to either contact your carrier for their APN settings or luck out with Google.
Link Here:World Wide Mobile Phone Settings