Many ISPs customize firmware for their networks. Visit your provider’s website → Support → Drivers & Downloads → search DSL2520UZ2 .
This server is maintained by D‑Link Russia and contains firmware for many ADSL models, including the DSL-2520U. dsl2520uz2 firmware free
Document your ISP username/password and PPPoE settings, as a firmware update might reset your modem to factory defaults. How to Install the Firmware: Step-by-Step Many ISPs customize firmware for their networks