I've just had to reflash my works HG612 as I couldn't get into the GUI - there was an over the air update years ago which had the side affect of disabling the GUI but telnet still worked so I never updated it.
Anyway I just flashed the latest version and still couldn't access the GUI on 192.168.1.1 then I remembered that flashing the firmware does not change the IP address or password. Changed to the correct IP address and now can login