Have openwrt installed on the device. If the sdcard is treated like a seperate storage space then have it automount in fstab and then have it mount as /opt and follow my AGH thread about installing and it will end up at /opt/AdGuardHome. [How-To-Updated 2021] Installing AdGuardHome on OpenWrt [Manual and opkg method] Just install like that once you have your device running.
Follow this for mounting your sdcard (yes it says usb drives but it will work for sdcards too. The mount device may be different however. usually something like /dev/mmcblk0p2
for an sdcard.)
Your /etc/config/fstab
will end up with an entry like this
config mount
option target '/opt'
option uuid '292f3823-ba8f-42bb-8821-f45cf5569341'
option enabled '1'
More info from i post i answered other day