I tried to update a new AR-750S to the latest version of U-Boot (https://github.com/gl-inet/uboot-gl-ar750-ar750s-x750/raw/master/bin/uboot-gl-ar750s-20181008-md5-3ca5683e0e40be42842051481bd072c6.bin) using the U-Boot webpage. I never received any conformation that the flash was successful or that it had failed, so after about 15 minutes I unplugged it. Now when plugging it in the power light glows dim and nothing else happens. Holding reset while plugging it in gives the same result. I have a UART connector on order. Should I be able to flash U-Boot via the serial interface?