A. CGNAT means Carrier-grade NAT and this is used a solution for IP address exhaustion.

Lots of Internet Service Providers(ISP) implementing this device in 3 network infrastructure.

 

This could impact on the stability of FEN connection on your client devices.

In this case, call local ISP and ask them opt-out from CGNAT Pool. Implementing a FEN relay service also can be a solution for this issue.