How to connect my rooted htc evo 4g to straight talk

I have a rooted htc evo 4g it is running digital karma enlightenment ROM, it is version 2.3.5 I have changed service from sprint to straight talk byop, but the service will not connect with my phone. Will not update prl. I think it is the ROM preventing It? it is rooted but says "s-on" so im sure it is something on my end. Any help would be great . Thank you