I had big issues with 1.3.6. version of the firmware. Upgraded to open wrt and then tried downgrade to 1.2.1. as suggested in David's post. After that the router was bricked.
Next I followed the in unbrick guide with tftp that David posted, but non of the firmware resulted in a successful unbrick.
But I got it to work, you need to find firmware 1.0.1. It's not on the tp-link site anymore, but someone posted it on 4 shared. this one successfully can be applied with this method. After that you can upgrade to 1.2.1. through tp-link ui.
Here is the link that I managed to find on the net for stock 1.0.1. firmware.
Hope this saves 4 hours of stress for the ones in the same situation 
Final addition. This stock firmware restore can also directly be done from 1.3.x versions of the router. Just use tftpd64 program and follow David's instructions and use the provided 1.0.1 version.
Just did this on my second archer c6 eu v2.0 without open-wrt