For telnet it's just gpon/gpon or gepon/gepon username/password to log in. The list or help commands will give you the list of available commands. Typing enable command will enable admin commands. Use the same password you logged in with to enable. Again, list will help you in a given context.
The super admin account applies to all fiberhome AN5506 ONUs as far as I can tell. Also adminpldt remains unlocked until the modem is rebooted and reregisters with the OLT. If you sign a waiver, pldt might grant you adminpldt access or simply switch your modem to bridged mode.