This method is not for beginners.
Using firmware intended for a different ISP or a slightly different hardware revision (e.g., HG6243C vs. HG6243A) can permanently damage your router. Once the flash memory is corrupted, the device will not boot, often referred to as becoming a "brick."
Navigate to > Device Information to view the current software build. Step 2: Where to Safely Download FiberHome HG6243C Firmware