Check your APN, check to make sure the proper mode is selected (settings - more network - mobile networks - network mode).
Make sure the correct settings are in the APN, it should be fast.t-mobile.com, WITH the DASH.
If all else fails, ODIN back to a full stock ROM and double check.
OR, give it a couple days. I get weird network stuff all the time. Things that make you think you really messed something up but turns out just to be a network glitch on their end.