Recommend always using a DNS name, and making it an Alias to your root server (not the actual server hostname).
The masthead file can only have one gather URL embedded in it. For the case of external clients registering through your DMZ relay, apply a _BESClient_RelaySelect_FailoverRelayList value during the client installation. You can add any number of Relays to the list, delimited with semicolons.
For the .EXE version of the client setup, see https://help.hcltechsw.com/bigfix/10.0/platform/Platform/Installation/c_windows_clients.html
If you’re deploying with the MSI installer, there’s a helpful tutorial on customizing the MSI at Edit the Bigfix MSI
If you’re using the Client Deployment Wizard, there’s an option to configure client settings if you expand the “Advanced” option.
Let us know if you need some help with any of that.