Yes, you can either provide a public IP to the codec or you can use a private IP and then have that translated using NAT (Network Address Translation) on the router and provide that natted IP on the codec in H323 NAT address and you should be able to connect to endpoints on public networks.
Here's the configuration which you will need to do on the endpoint:
xConfiguration H323 NAT Mode: On
xConfiguration H323 NAT Address: X.X.X.X
Where X.X.X.X is the natted IP address setup on your router.
You just need to specify the NAT address which is configured on your firewall. This is present so that whenever IP packets are sent from your device, the packet headers as well as the information inside the packets contain the same IP address and you may not face Audio/Video issues.
Please try this and let us know if you face any more issues.