As of 2023, the DSL-2877AL . The Broadcom BCM63168 chipset lacks open-source drivers for the DSL modem and hardware NAT. Attempting to flash non-D-Link firmware will permanently brick the device.
However, I can provide a that explains the concept of firmware for legacy DSL modem-routers (using the closely related DSL-2877AL as a hypothetical case, or the real DSL-2877B / DSL-2730U series as reference). This will give you the framework to substitute the correct model once you verify the number on the device’s label. Dsl-2877al Firmware
D-Link has EOL’ed (end-of-life) this model in many regions. Do not expect new security patches after 2024. For critical networks, start saving for a replacement (e.g., TP-Link XZ000-G3 + a separate router). As of 2023, the DSL-2877AL