From your comment, it looks like the FQDN for the machine hosting SEPM is not the same as public DNS for the server.
One of the possibilities to allow clients to connect to SEPM using public DNS is to create a Management Server List with public DNS name as priority 1 entry. You can create a new Management Server List in the Policies tab, under Policy Components, and then assign it to the group of clients. For more details on how to create and assign management server list, please refer to the following URL -
http://www.symantec.com/business/support/index?page=content&id=HOWTO55402
Of course, now as you have mentioned this information will be exported along with the client package. So need to export the client export package for the group (to which MSL is assigned) and install the client. For already installed clients, you can probably export sylink.xml for the group and import this in the clients.
Check this thread
https://www-secure.symantec.com/connect/forums/how-does-one-configure-sepm-121-manage-out-network-computers