DASAN H665 has vendor backdoor built into BusyBox /bin/login. Account named “dnsekakf2$$” gives access to admin (uid 0) account over telnet, at least for administration interface documented in H665 QIG (Quick Guide).
DASAN H665 is GPON Optical Network Terminal (ONT). It could work as router/NAT or bridge and allow End User to Web Access device at http://192.168.55.1/. Depending on device’s configuration it might assign different 192.168.X.0/24 subnet over DHCP, but device still reply to traffic on 192.168.55.1 from any devices in 192.168.55.0/24 subnet.
This vulnerability was assigned CVE-2019-8950.