According to the manual the D-Link DSL-2520U ADSL2/2+ router has default admin credentials:
Connecting to Web-based Interface
When you have configured your computer, you can access the web-based interface and configure needed parameters (for example, create an interface to connect to the Internet, specify the settings
of the firewall, edit the password for the administrator account, etc.)
On the opened page, enter the username (login) and password for the administrator account (by default, the following username and password are specified: admin, admin). Then click the Enter button.