The Zyxel Prestige 782R router (Shodan search) suffers from the RomPager /rom-0 bypass mentioned on earlier blogs.
If you request the /rom-0 file it does not require authentication. This can be reversed using available tools like the one at http://188.8.131.52/zynos.php. The first string returned is the admin password.
Ultimately this is due to the router using the RomPager server. Can identify from the header: