iPhone 3GS stuck at "Searching"

[Sysop33]

iOS Version: 5.1.1
Modem Firmware: 05.16.08

Jailbroken with redsn0w and latest version of ultrasn0w successfully installed.
It correctly displays the carrier, but there's no network available. I'm quite sure it's not blacklisted, it used to work. I installed 6.1.3 by mistake and I had to downgrade it to an older version, I used a custom IPSW (basic IPSW combined with SHSH blobs), everything's fine, apart from what I've stated above.

 

[Sysop33]

Nvm, this baseband version is not supported

 

[Applejuiced]

Nvm, this baseband version is not supported

There you have it.
If its an AT&T iphone 3GS you can unlock it by purchasing an imei unlock for a few dollars.