about sd boot problem read there Final release of OpenWrt 19.07.8 for Khadas SBC VIM1 VIM2 VIM3 VIM3L Edge - #75 by hyphop
Just an FYI regarding the youtube video showing how to install OpenWrt. They show the address to access via wifi as openwrt-vim. That is not correct.
maybe video have expired information 
wifi connection is open, connect, then
ssh into the VIM at root@openwrt-khadas
must works !! what a problem ?