As a consolation, your Odroid H2 could be a useful 24/7 workstation, running Ubuntu, openSUSE etc, that can boot in Secure Boot mode.
It is technically possible to create your own Secure Boot key. see:
http://rodsbooks.com/efi-programming/why.html
I try to avoid purchasing any IT device for which I can’t first download a fairly complete manual - in order to avoid the “gotcha” that you have encounrtered. “Economy” products tend to be particularly light on documentation.
I did specutatively purchase an AMD mini-PC, that I reported in another thread. As a routine openSUSE user, I would have been able to re-purpose it, should Secure Boot not have been able to be disabled. Mini PC Hardware
Arguably any SBC having Intel CPU is not “great for a firewall appliance”. Lightning Wire Labs mini appliance has an AMD APU.