If you had googled “rock 4 se public key not available” then the first result would have been this thread that contains the solution to fix that error -> Sudo apt update get error
export DISTRO=focal-stable
wget -O - `apt.radxa.com/$DISTRO/public.key` | sudo apt-key add -
apt update should work fine after those commands.
I have abosolutely no idea about FEX beyond that. This just gets apt update not moaning.