Unless you are in control of the top level hosting account called WHM on your hosting server, you may not be able to adjust the whitelisting of our IP's.
Without whitelisting DKIM, SPF and DMARC will break. The workaround to this is to enable SMTP authentication so that the Gateway authenticates to your Cpanel server when delivering emails. The authentication will use an email credential.
In Cpanel please create a new users, and example could be firstname.lastname@example.org and then use these credentials in the Gateway setting in the DuoCircle portal.