The main domain and the add-on domains need to be on different dedicated IPs.
This cannot be done directly through cPanel/WHM .
If suppose your cPanel account username is “USER” and domains are:
Then you have to edit the following files using vim or pico editor :
And change the IP field to in each to your desired one.
Then you have to rebuild apache using /scripts/rebuildhttpdconf
Restart apache service.
Edit DNS zone file of each domain and point A record to the new IP.
That should be it! All your domains should work flawlessly and you should also be able to install SSL , if required.