Since you can load OpenWrt, use it to sysupgrade a stripped stock firmware, and it will be back to stock. It is a good idea with any revert to stock that once the stock firmware boots, use it's web page to revert to factory settings and then again flash the stock firmware unaltered.
Download from TP-Link and unzip on your PC. Do not try to flash the zipped file.
If the stock firmware file does not have "boot" in the name you only strip the first block of 512 bytes. If it does have "boot" in the name you need to strip 257 blocks of 512 bytes.
The strip can be done in place on the router's RAM disk (/tmp) with the dd command.
dd if=orig.bin of=stripped.bin bs=512 skip=1
sysupgrade -n -F stripped.bin