Official firmware can be sourced from regional D-Link support sites: Wireless N 300 ADSL2+ 4-Port Router DSL-124 - D-Link
If your version is lower than 1.04, and you are experiencing issues, an update is recommended. D-link Dsl-124 Firmware
A modern replacement like the D-Link DSL-225 or a separate VDSL modem + router combo would be far more secure. Official firmware can be sourced from regional D-Link
Security and maintenance best practices