Yeah. This is my problem too. Long time without touch these legacy devices, and I don't remember some things either. Thank you for your answer. I can't mark it as a solution, but thanks anyway.
This is true of most of the TP-Link of that era. Download and unzip the factory firmware (which should have "boot" in its name) and strip off the first 512*257 bytes (128 kB + 512 bytes) to create a stripped file. Use this file as a sysupgrade in OpenWrt-- make sure to not save settings and you will have to force flash.