Before starting, ensure you are downloading the correct file for your specific hardware version (e.g., HW I1 or HW R1), as installing the wrong version can damage your router.
(e.g., A1, B1, T1), which can be found on the sticker at the bottom of the router. Official Support Page: D-Link Support Portal D-Link Global Download Center D-link Dsl-124 Firmware Update Download --BEST