The problem is not really a DNS problem. The problem is more like not reading the manual properly. You could have read that you have the option to fill in an external address/IP for the specific sub-domain you desire.
In the server manager you can let the www section point to an internal server but also an external address. This is what I do for years with SME already and never ran into trouble.
So... don't let the
www.domainname point to SELF but to the IP address from the Hosting Provider.
Another option that can be used is a simple HTML file with a redirect in the Primary section to achieve the same goal.