It's probably ignored the same way as e.g. on the Mi Router 4A. You'd need an SPI NOR flash programmer and then modify uboot to make it accept UART-input, like e.g. I detailed in Observations on Xiaomi MIR4AG newer firmware
Your uboot is unlikely to be the exact same as mine, so exactly what you need to change is unlikely to be the same. Also, without a programmer, you are basically sh*t out of luck.