Hi friend.
DG8245W2-10 is a carrier product.
Usually, the administrator password is used by field engineers to debug the equipment. If you really need it, contact the device vendor to obtain it. Users typically only need to know the commonly used username and password (on the nameplate on the back of the device).
You cannot change DNS settings, most likely because your ONT is customized by the carrier. Your ISP restricts the use and changes of this feature, and it is recommended that you contact your local ISP for assistance.
Thanks.