Looks like your keyring is not up to date.
Is your system up to date? what does yay -Syyu say?
Which keyring version have you installed? pacman -Q | grep keyring
Correct, this is an ISO install using the online install, which I have installed from once or twice in the past using this very same usb stick.
I ran the installer multiple times (3-4 I believe) with the same outcome, so multiple rankmirror attempts all with the same outcome.
I even attempted to update the keyring attempting to both update the key’s with (–refresh-keys) and updating the individual packages archlinux-keyring & endeavouros-keyring. Both were unsuccessful.
Only way I was able to install was to do an offline install and updated afterwards.
Edit: to answer your question directly, I did not attempt to update the mirror list outside of the installer, as I felt it was redundant. If I need to re-install and the problem has not been identified, I’ll try that.
This is always recommended in case you have issues with getting packages or slow download while installing the OS, the installer is ranking internal, but thats not perfect in all cases, doing it before starting the installer is known to resolve most of such issues.