Only download firmware that matches your Hardware Version and your ISP . A 2021 file for ISP "A" will NOT work on ISP "B."
Since you cannot find this on the official FiberHome website, use these three alternative methods: download firmware fiberhome hg6243c 2021
Once telnet is open (Username: root / Password: hg2x0 ), you can dump the mtd partitions. Only download firmware that matches your Hardware Version