While I understand that you would like to get a quick fix, creating several threads with the same content isn't going to help.
You need to get the source code of firmware-utils from https://git.openwrt.org/?p=project/firmware-utils.git;a=summary and change tplink-safeloader.c to use the correct offfset. Then you recompile firmware-utils and run the changed tplink-safeloader.
As an alternative (because it might be easier to compile, I never tried compiling just firmware-utils) you can check out the complete OpenWrt source tree and compile firmware-utils from there. This is explained in https://openwrt.org/docs/guide-developer/toolchain/use-buildsystem
As yet another alternative you could ask and hope that someone creates a sysupgrade-flashable firmware image for you.
And as the last alternative you can still decide to stay with OpenWrt and run plctool manually or on another computer. This would be my personal choice.