first step restore iOS 5.1 using custom firmware with old baseband preserved method by redsn0w redsn0w_win_0.9.10b6 then jailbreak and install ultrasn0w_1.2.5.deb and mobilesubstrate_0.9.3901_iphoneos-arm.deb through cydia or ifunbox then go to cydia and do the following Step 1: Open cydia go to manage, sources, ‘Edit‘ option in the top right hand corner of the screen before selecting the ‘Add‘ button on the left hand side of the tool bar. Step 3: In the pop up alert view enter the repository URL which is <Bạn vui lòng đăng nhập hoặc đăng ký để xem được nội dung này!> , then press ‘Add Source‘. Step 4: After the repository has refreshed and finished loading sources, tap on the repo name in the sources list which will display the Ultrasn0w Fixer utility. Step 5: Select the Fixer utility, pressing ‘Install‘ in the top right hand corner. Confirm the installation and then wait for the process to be completed. reboot the device and that all the phone is now unlocked
