Quick answer: CGNAT means your ISP shares a public IPv4 address among multiple subscribers. Your router can therefore have a different WAN address from the public IP websites see.
Why ISPs use CGNAT
IPv4 address space is limited, so carriers use additional NAT layers to serve large numbers of customers without assigning every connection a unique public IPv4 address.
How to spot it
Compare the WAN address shown by your router with the address returned by WhatIP. If they differ and your router has an address from a shared or private range, CGNAT may be involved.
What CGNAT can affect
Inbound connections, port forwarding and some self-hosted services can be harder behind CGNAT. It does not mean the connection is broken; it is a network design choice.
For the mechanics, see What Is CGNAT? and What Is NAT?.
Want to see this in practice? Check your own connection's IP, location and ISP.
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