you wont be able to access the settings of that modem router. your best bet is to call PLDT and have one of their technicians change the SSID or ask them if you can have the login info of the router so you can set it yourself (although I doubt na ibibigay nila ang details)
OR you can just plug in a separate wifi router and you can set whatever SSID you want and connect to that router instead of the one given by PLDT