We have a very easy way of unlocking the ATT HTC One A9. You will be able to convert your ATT A9 to the USA Unlocked A9 running 1.12.617.6 version
To unlock AT&T HTC One A9, you will need three things. 1.12.617.6 RUU that is downloadable from here (direct download link from HTC’s website).
2) Drivers, S-Off and bootloader unlocked from HTC dev’s website
3) ADB.
Unlock ATT HTC One A9 steps
step 1: boot into download mode using following command. This is the black screen that you can get to from the bootloader. You can also boot into download mode by turning off the phone, and holding the power + volume down buttons until you see the htc logo.
adb reboot bootloader
step 2: Change CID. Type this.
fastboot oem writecid BS_US001
step 3: if your mid is something other than 2PQ912000 you can change it with the command
fastboot oem writemid 2PQ912000
step 4: Now type in this:
fastboot reboot-bootloader
step 5: now run the RUU’s setup that we downloaded earlier. Run it and flash the ROM. You can now install HTC One A9 custom recovery and root it using super SU.