exiting news! We made some progress and got the phy working 
Setting the phy register to the correct value for the rtl8211s was the solution.
Also we adjusted the partition scheme to fit what we saw in the oem bootlog
see our wip changes here
and just after we found that out tp-link answered and published their sources 
@svanheule just for curiosity, how do you include the mdio-tools in the openwrt build? I couldn't find a package for it...
now I try to flash a squashfs image, but I wasn't successful yet