Short answer
CGNAT means Carrier-Grade NAT. It is NAT at the internet provider, not only in your own router.
Why providers use CGNAT
Because IPv4 addresses are scarce, providers sometimes share public IPv4 addresses between many customers. This reduces the need for unique addresses.
How CGNAT appears
You may see a private or provider-side address on the router WAN page while VisaIP shows another public address. Incoming connections may also be hard to make work.
What you can do
If you need to reach a home service from outside, ask the provider about public IPv4, fixed IP, IPv6 or use a tunnel/VPN solution.
Common questions
Is CGNAT bad?
Not for normal browsing, but it can interfere with port forwarding, home servers and some games.
Does IPv6 solve CGNAT issues?
IPv6 can provide direct addressing without CGNAT, but firewalling and proper configuration are still needed.