I recently purchased a used SM-G900FD that worked with both TMobile and AT&T when I looked at it. I checked the IMEIs but then things got weird.

I wondered why it was still on Kit Kat and wasn't updating OTA, so after some reading I downloaded Phone Info. I noticed two things, S/N was 00000000000 and it was rooted. So I downloaded Odin, and (I think I) downloaded the correct firmware, G900FDXXS1CPG1.

This gave me 6.0.1 and the phone info now says it is not rooted but I can't get service from AT&T or T-Mobile. Is this the correct firmware? I read about baseband versions but I don't know how to download the correct one. I am back to the working (4.4.2) firmware, which is G900FDXXU1ANJ1.

Other than selecting the correct country code, which is CAC, how do I know I am downloading the correct (baseband?) firmware?

Thanks in advance.