How to get fastboot identifier token to unlock bootloader on HTC One M9 where Android is no accessible?

I want to revive an old HTC One M9 rotting in my grandmothers locker for some years. I would like to flash the phone with a custom ROM but unfortunately it is locked, so I can't access Android. However according to the HTC documentation to unlock the boot loader on has to enable developer options in Android settings first: > you will need to first manually enable OEM unlocking in Developer > options in order to allow the bootloader to be unlocked and proceed > with the next steps. Apparently without doing so it is not possible to get the identifier token which is needed to unlock the bootloader. fastboot oem get_identifier_token (bootloader) [KillSwitch] : /dev/block/bootdevice/by-name/frp (bootloader) [KillSwitch] Last Byte is 0X00, disable unlock (bootloader) [KillSwitch] oem unlock Turn Off! OKAY [ 0.063s] Finished. Total time: 0.063s Is there any way to unbrick this phone? UPDATE: =========== As suggested in the comments I tried to flash > the original RUU ROM to restore a proper state I got it here: https://htcstockrom.com/htc-one-m9 The following steps are suggested: - Enter: fastboot oem rebootRUU - Now enter: fastboot flash zip update.zip - Once the installationis finished, reboot the device. However fastboot oem rebootRUU makes the device humming and blinking but does not lead to a status where I could successfully upload the file (same problem described here ): https://www.youtube.com/watch?v=VVG4kT-LiQQ&feature=emb_rel_end I can upload it in normal download mode but there I get: FAILED (remote: '9 RU_SECURITY_FAIL zip from usb command in download mode') fastboot: error: Command failed