Unable to access dashboard as Super Admin using Multidomains

I have written a plugin which simply redirects all users other than the superadmin to the front-end of the site, blocking out wp-admin. This works fine on all sites using the primary domain. However, when I create a site using one of the multidomain urls, and then go to its wp-admin as superadmin, it redirects me to the login page.

here is the code that i'm using for the redirect:

add_action( 'init', 'level_check' );

function level_check() {
if (is_admin() && !is_super_admin() && strpos( $_SERVER['REQUEST_URI'], '/customize.php' ) === false ) {

header( "Location: /fmm-crm/");
die();
}
}