With the recent release of iOS 5.1, not only the jailbreak exploits were effected but also those who had unlocked their devices got affected by Apple’s latest iOS release. Luckily there’s a tweak available on Cydia by the name of UltraSnow Fixer which regains the unlocked status of A4 iOS devices.
Following are the steps for this process and before starting the process, be sure to have in e of these baseband version : 01.59.00, 04.26.08, 05.11.07, 05.13.01, 05.12.01, 06.15.00. (You can check this by going to Settings > General > About > Modem Firmware.)
How To Unlock A4 Devices Using UltraSnow Fixer
STEP 1. Ultrasn0w Fixer isn’t in a default repo, so you’ll have to add it. To do this, open up Cydia > Manage > Sources > Edit > Add, and type in the following URL:http://repo.iparelhos.com.
STEP 2. Once the repo is done loading, search Cydia for Ultrasn0w Fixer. Locate it and install it like a typical package.
STEP 3. After the Fixer is done installing, you need to grab the latest version of Ultrasn0w. This is in a default repo, so all you have to do is search for and install it.
That’s it your device is unlocked and good to go. On the side note, it’s worth knowing that the iOS 5.1 jailbreak is not completely stable so it might cause some unknown problems and the official ultrasnow version is expected to release soon so unless you don’t need to unlock you iOS device desperately, wait for the official tool for unlock.