I also have the TP-Link Archer C50 (ES) V6.0 device. I tried using a V4 image using the approach here, using the Web UI to load it, but it rejected it.
However looking at the TP-Link official image files, they are completely different for the EU and ES versions of the device. The EU image BIN file is 7930368 bytes, and the ES image BIN file is 2183572 bytes. So I think they're ripping us off in the ES regions and selling a cut-down device. Maybe it even has less flash and/or RAM, which would likely make it useless. So I'm giving up with this device. I'll use it as an access point elsewhere and look for some other router to put OpenWRT on (which I need for load balancing and monitoring) -- unless anyone has any other ideas.