"myDlink.dyndns.org uses an invalid security certificate. The cerificate is only valid for myWHS.HPshare.net"
Yeah, that's normal with such certificates having been issued to work on a specific site only. You would need to have a certificate for myDlink.dyndns.org too, or you would setup a WebHop from myDlink.dyndns.org to myWHS.HPshare.net (only if the latter can be accessed by this name from outside...). Or you would have to get your browser not to check certificates...
Using the myDlink.dyndns.org, I can connect with and log on to the myWHS.HPshare.net, which has the TZO issued DDNS. That is obviously not only wrong, but perhaps dangerous as well.
Is this via HTTP port 80 or via HTTPS port 443? If it is via HTTP, then this is normal: certificates are not checked or used at all with normal HTTP. If you don't want this type of connection, simply don't open port 80 on your router.