How Should TCP/IP be configured to allow an external application access to SQL via the same port #?

My SQL Server instance is hosted on an internal network server (10.x.x.x address). It's a named instance currently using dynamic ports. An external application at a third-party hosting location connects to the instance through use of the IP address and port #. After each maintenance window I have to confirm the dynamic port # in use is the same as before as the firewall rule is set to allow connections between application host and database host on that specific port #. Is this the correct way to ensure the externally-hosted app can connect to the host without having to change port numbers in the connection string or should another port forwarding method be used (i.e., port 9999 on application host is mapped to port 1434 on SQL host)? What configuration manager changes in the Network Configuration section should be applied to allow the external application to use the same port # but internal applications to rely on the Browser server to make the correct mapping?