Portal link for URL are not resolved for external users signed on to UCS portal

Setup portal link on the start site of the portal. The setup URL in Link references to an internal application server. The URL is resolved by the internal company gateway server (firewall and dns).

Adding the internal IP address works for internal users, already connected to the same domain as the UCS server. External users from outside the company cannot connect to the application. The internal ip address is not resolved by the UCS dns server. The internal ip address is only resolved for internal users.
Adding the server host name to LDAP on the UCS server does not sole the problem. The server itself can validate the DNS entry and ip address as per nlslookup

I had the understanding that once the user has signed on to the portal, the UCS server will resolve all URL links in the portal link with the internal UCS dns server and if the server is not able to resolve the address then the external dns server is called. Why are internal ip addresses or URL not resolved by the Internal UCS DNS server or external signed on users?
Thank you for your response.