Glossary
What is CGNAT (Carrier-grade NAT)?
CGNAT means your internet provider shares one public address among many customers. You cannot receive connections from outside, so port forwarding does not work.
Public IPv4 addresses are scarce, so many providers put several customers behind one. From the internet your connection has no address of its own, which breaks any setup that depends on reaching your router.
Cameras behind CGNAT can still send video out. That is why a device that connects outward to a server, as the LunaCam Box does, works where port forwarding fails.
If a provider offers a static public address for a fee, that also solves it, but it costs more and exposes the DVR directly.