Hi,
The first thing you can try is flashing a normal firmware from the TP-Link website to see if it's the router that's refusing the debug firmware.
Despite getting the router upstream, I'm not the most knowledgeable person in this forum. I don't know the inner workings of the stock firmware.
Other people have also reported issues with flashing the device (possibly with newer firmwares?). I am not sure why this happens, it could be that TP-Link somehow managed to block it.
What I do know is it can't be the bootloader at fault here because u-boot upgrades are not enabled.
If the current method doesn't work on newer firmwares, an exploit could be used. Over on the S4 forum, the person who made the PR for the router found an exploit in the bootloader which enables TFTP temporarily which then pulls and flashes a firmware without any checks.