0KTAV1US

Hello, you can only boot from sdcard with that device if you follow the rockchip boot sequence, ie: you have to pack u-boot and trust.img using loaderimage tool from rockchip rkbin repository.
 
That tools decorates the uboot/trust.img binaries with some signature and checksum, then you have to put on specific positions on your sdcard and the miniloader, residing in the emmc, will boot from sdcard once it validates correctly the binaries you supplied.
 
You may take a look to my multitool github project for some reference. I have you same box and it boots on mine.
 
Unfortunately on this particular box, the manufacturer disabled the sdcard boot at SoC/hardware level: this means that the trick to erase the internal flash to boot from sdcard, which worked fine for older SoCs, does not always work with rk3528-based (and probably other rk35xx) boards