You can see RouteOS version installed on the “unresettable” device?
I still have a previously configured WIFI without an IP on the device, which I try to correct.
It mean you configure THIS device, but you lost access to it?
Or ANOTHER device?
“device is then just simply rebooting the previous faulty config”
if someone has activated the protected routerboot, you can not reset the device on unusual way,
you must press the reset button for exactly 300 seconds (start when regulary blinks 1sec on / 1sec off) and wait 3/4 minutes for format internal FLASH,
then is visible on netinstall.
And is possible than someone set the latest reformat-hold-button-max to different value, you must only guess the right time from 10 seconds to 600
more detail:
http://forum.mikrotik.com/t/problems-getting-mikrotik-to-work-with-dsl/442/1