Frustrating 3g issues!

[rjgarment]

I have a Nexus One with T-Mobile. My brother-in-law worked for Google and his coworker gave him a new N1 to give to me. I cracked the screen, and he sent me another one, but it was having issues with the sim card causing overheating and constant reboots. What I did was switch the working mainboard from the phone with the cracked screen to the phone with the good screen (and bad mainboard.)

The phone works perfectly except I can't get a 3g signal. When I'm on WCDMA preferred, I can see AT&T and T-Mobile under network operators, but when I switch to WCDMA only, I get no data signal and can only see AT&T under network providers. What gives? I realize that the new phone that had mainboard problems might have been an AT&T phone, but switching my mainboard out should have fixed it.

Either way, since they both use band 2100 for 3g, why can I not see the T-Mobile network when I switch to WCDMA only?