2.19.40.20 is also unlockable

Bootloader of both ZE550ML and ZE550ML can be unlock if rom is on 2.19.40 or 2.19.40.20

How to unlock bootloader:

ZE551ML OTA 2.19.40 OTA Download:

Download ZE550ML 2.19.40.12 complete Rom:

Download ZE550ML 2.19.40.12 OTA Rom:

Tip:

white splash screen for 551

Twrp Flashable, Black and white splash screens for 551

Congrats to all ( both ZE551ML and ZE550ML users), bootloader has been unlocked without connecting to ASUS servers.New update 2.19.40.20 can be unlocked as well and Asus did not patch the hole. Thanks to @ oxydius7 for confirming this here Lets discuss, problems and unlocking related experience here.1. Update to 2.19.40.18 or 2.19.40.202. root it3. make sure adb is insatlled and working4. enter the following commands in cmdadb shell (press enter)su (press enter)getprop ro.isn > /factory/asuskey ( press enter)reboot bootloader ( press enter )Phone will restart in bootloader and after updating itself phone will restart again and ASUS splash screen will be white ( locked bootloader has black screen, you will see difference by yourself)5. download black splash screen for 551 from hereor for 550 from here6. navigate to adb folder open cmd and type the following commandsadb reboot bootloader (press enter)fastboot devices (press enter)see if device is listed, write followingfastboot flash splashscreen splashscreen.img (press enter)restart your phone and now black splash screen is back. Congrats your bootloader is now unlocked.If you follow my steps therefrom internal memory and sd card, but i will advice to take a backup in case if you are out of luckyou guys can side-load or flash in via recovery methodThanks to @ fuzzybuffalo for providing the linkcredit goes to @ keepfishes for coming up with links on post credit goes to @ singhnsk for coming up with links on post if you have problems with adb try to install "Terminal Emulator" app and running the commands from theresugetprop ro.isn > /factory/asuskeyreboot bootloaderThanks to @ hugoprh for providing the tip, thank to him on his post credit goes to @ fuzzybuffalo for providing the link, thank to him on his post If you want twrp flashable zips of both white and black splash screens of 551, please download them from here All credits goes to @ shakalaca for providing unlockdonate and say thanks to him, press thanks button if i helped