Just to be clear, the reason you cannot use the URL to access the CCTV from inside your network is because the URL resolves to the external IP address of your router, but routers do not loopback connections to inside the network.
There are two ways to resolve this:
1. Use the IP address of the CCTV when you are inside your network.
2. Modify the hosts file on each PC inside your network to add the URL and internal IP address. Windows uses the IP address in the hosts file first.