We don’t support any OSes beyond their general EOL. The only way to run RHEL 7 safely at this point is to pay for ELS.
But, we also won’t break it any time soon (though it might break before the four years of ELS, we aren’t testing on RHEL7 anymore). If you switch to our newer repos, you can keep getting updates to Virtualmin packages, but you won’t ever receive an update of the httpd package, as I’m no longer maintaining custom ones. You’ll need to switch to the RHEL httpd package, and you’ll need to switch to an execution mode other than fcgid+suexec. (PHP-FPM is the recommended mode for everyone in almost every circumstance, and switching to it should generally be pretty painless).