Cannot connect to USA GSM Towers...please help.

[TheKid1]

Ok, so this is what I'm did. I dropped Sprint and went to cell phone store for cheaper unlimited plan. The network my phone will be on is called Simple Mobile which is a 3rd party of Tmobile/At&t towers. Everything is set up for this new network. They unlocked a SIM card but they said I need to flash something so that I can connect to GSM towers in the US. I don't want to mess my phone up so here is what is done to my phone so far so someone can steer me in the right direction:

HTC TP2
SIM Unlocked (the cell phone store purchased code)
Phone Unlocked through CDMA
Running Vin's Plutonium ROM.

Please help. Is this an easy fix? I installed the ROM myself, so I'm getting better at this stuff, but I am still a little confused about all this, so I ask that you guys be clear as possible about this. Thank you so much for all the help I have received so far.