The 660HW-67 was distributed in Germany as the "WLAN Modem 100" through the Arcor ISP. It came with the firmware
which seems to be originally made for AOL.
To get the router ready for ADSL2+, I needed the
QQ.7 firmware which is the original one.
The big problem is that the
rom-0 of the original firmware is 48 KiB whereas that of the AOL firmware is only
16 KiB. Usually, you make an upgrade by updating the
rom-0 file (which contains default settings) and then update
the firmware itself which then reads the new default settings upon the next boot. Since the router didn't accept the
new settings, it stuck after the reboot.
This is how it works (using the serial connector on the PCB and a terminal program):
rom-0doesn't match and ask you to upload a new firmware