It's difficult to say without knowing the actual domain name. Could you clarify the exact method you used to add the CNAME entry? What shows up in the Apache error log?
If you have root access to the server, feel free to open a ticket if you would like for us to check further:
I see no issues with the configuration as you have described. Do you have root access to the server? If so, I suggest opening a support ticket so we can check further:
Unfortunately not. It may be a good idea to explain my application just in case what I want to achieve isn't possible anyway.
I want to host an application on a primary domain which is used by multiple resellers. A reseller's site will be something in the form of reseller.primarydomain.com.
The reseller will be able to BYO their own domain and CNAME it on top of reseller.primarydomain.com.
A PHP script will be able to associate the CNAME record with the reseller.primarydomain.com.
I see no reason why you would not be able to achieve such a setup. I suggest contacting your web hosting provider to determine why the CNAME entry is not working properly.