add on's domain name is actually registered at another IP address
This statement is rather confused. If the authoritative DNS Zone is on your WHB server, then that is the zone that is required to contain the information about the addon domain. Otherwise DNS lookups won't know where to find that addon domain.
I suggest that you clarify what you have done with the setup, and also tell us both the domain name and the addon domain name.
I am not WHB staff and I am not paid.
I provide help in these forums on my own time.