Because you are given private IP, like mobile internet fromndogo/maxis/celcom all are private IP.

So far I know only unifi give public IP by default

If don't have NATing, or public ip access from external, you need to use hosted services where your cctv will be permanent connect to server online. When you need to access, go online to the server and view your cctv realtime, the server will bridge external to the cctv.

Like how Xiaomi DaFang cctv work.