rerouting subdomains without cpanel

I own two domains, lets call them domain.com and domain.org.

I’m trying to route visible.domain.org to realsub.domain.com
Wordpress installation is on domain.com, while I have established a subdomain in realsub.domain.com, which showed up correctly with at this address – that is before the attempted redirect from visible.domain.org.
To be clear: I want visible.domain.org to display the site located at realsub.domain.com. So I set up routing in the registrar control panel of visible.domain.org to domain.com (in fact its ip).
In wordpress on domain.com I have installed wordpress-mu-domain-mapping.php and sunrise.php, created a new domain “visible.domain.org” with the same ID No as the “subsite” realsub.domain.com.
No matter if visible.domain.org is checked as primary or not in wordpress, it displays the error page “domain is parked at subsys”–which is the registrar of both domains.
However, if visible.domain.org is unchecked as primary in wordpress, realsub.domain.com displays the correct site.

If I don’t misunderstand the documentation, visible.domain.org should be parked in Cpanel. However, subsys doesn’t offer Cpanel, and I can not find any setting for this.

Anybody got a hint for me?