Is there a carrier unlock exploit?

[Gubba-Monster]

Sorry I am a total noob when it comes to jailbreaking. I have this old iPhone 3gs lying around that is carrier locked, and for 2 years I have been trying to figure out how to unlock it. I have already followed the jailbreaking guide (in this repository), and am currently running iOS 5.1.1 with downgraded bootrom (I think thats what it is). Could someone explain to me how to bypass the activation screen so that I can unlock it from the main "desktop"/UI

Any help is appreciated. Feel free to ask questions! Thanks, Karsten