On some low-cost STBs, the boot mode pins (e.g., BOOT_SEL) are set by pull-up/down resistors. If a resistor is missing or a solder joint is cracked, the chip might boot into UART recovery mode by default.
This is the most successful fix for 80% of users. The Bootrom is waiting for a specific byte or sequence over UART RX. You need to send it. On some low-cost STBs, the boot mode pins (e