The HG6243C is a MIPS architecture device. Most modern "TOP" or "Ansuel" firmware packages are developed for ARM based FiberHome devices (like the HG6245D or AN5506-04-FA). Installing ARM firmware on a MIPS device will permanently brick your router.
: Official firmware files are rarely hosted for public download to prevent users from accidentally "bricking" their devices with the wrong region's software. 2. How to Access Your Router's Admin Panel download firmware fiberhome hg6243c top