If the box is dead (red light only), use a CH341A or similar SPI programmer to flash the 8MB chip directly.
Here is the forum-style post for the firmware dump. Subject: [Download] Geant GN-HD90 Original Firmware Dump (ZIP) Hello everyone, Download Dump Original Geant HD90 zip
Flash this at your own risk. Ensure your power supply is stable during the process to avoid permanent bricking. If the box is dead (red light only),