[HELP][CRITICAL] stuck with no recovery, no root and locked bootloader on Sultan CM13

Hello Guys,
I was trying to install OxygenOS 3.1.0 when I was on Sultanxda's CM13 but instead bricked the device.
What I did:-
1. Rebooted to TWRP and flashed stock OOS recovery and then clicked reboot into recovery.
2. Instead of rebooting in recovery my phone got stuck at the OnePlus logo.
3. I booted into fastboot to flash twrp.img but it says 'writing 'recovery'...
FAILED (remote: device is locked. Cannot erase)'
4. I repeated the same with stock recovery but no success and was not even able to boot to CM13 now.

Hence I was stuck on the OnePlus logo without any booting ROM or recovery and a locked bootloader (according to fastboot)

What I have already tried but failed:-
1. Tried to root with kingroot etc. (Before bricking)
2. Tried to unlock bootloader but failed because it has no recovery.
3. Tried to flash stock OOS 2.1.3-x
4. For now I used fastboot continue to boot back into CM13

Please help, I'm now stuck with cm13 without root and without any recovery installed.
Thanks.

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