You will need to rework R33, R34, R38, R39 on the MaaXBoard. Out of box it is setup in internal boot mode (SD card). If you wat Serial Downloader mode rework is required.
Also of note, the MaaXBoard AES-MC-SBC-IMX8M-G by default comes with SD Card populated, not eMMC. Typically we reccomend development to be done with a version with SD card, and then transition to production w/ eMMC.