Default [Q] Issues with Google ROM and Radio

Hello,

I'm having some trouble getting a custom ROM to function properly. I've tried two different Google based ROMs and I no signal whatsoever. I have a friend who has the same phone, same provider and country and can get these ROMs working fine. He suggested that it is due to my older modem version and should consider updating it. I was told I could simply revert back to the stock Samsung ROM and do a Software Update and it should grab the new modem firmware but when I tried to do this, it told me my phone had been modified and I could not receive updates. How can I update the modem version to the newest without having to restart from an unrooted factory default phone?

Any suggestions and help are appreciated

Thanks