Can you check the installation in eMMC on K2 ?
After recording the Armbian image for the TV box (add u-boot to SD card and DTB settings), you need to download and copy these two files to the /root directory on the SD card. After starting the system from SD card to K2, you need to run the script s905_install.sh from the user "root". The script should install the system on eMMC and create a new u-boot (special version for eMMC). I have on S905 this works (I installed in eMMC the latest armbian image with kernel 5.3.0).
Before starting the system SD card and run install on K2, to avoid side effects, you need to completely clean the eMMC.
If the u-boot installation does not work from the script, you can re-write the file (uboot_s905_emmc_1m. img) to eMMC after the script runs s905_install.sh two commands
dd if=uboot_s905_emmc_1m.img of=/dev/mmcblkX conv=fsync bs=1 count=442
dd if=uboot_s905_emmc_1m.img of=/dev/mmcblkX conv=fsync bs=512 skip=1 seek=1
https://yadi.sk/d/3RhEo60liJuTDw
For a quick check, you can immediately write uboot_s905_emmc_1m.img to eMMC with a single DD command and check the overall launch of the u-boot itself with eMMC. After writing u-boot to eMMC, there should be a partition table (two partitions) on eMMC.