CG-NAT (Carrier Grade NAT) is when instead of giving you a true Public IP address, the ISP has given you an 'private IP'. It allows the ISP to NAT multiple customer's onto a single Public IP address. One of the limitations of this is that end customer's cannot perform port-forwarding, as they do not have a dedicated Public IP address for their service. You will need to contact the ISP to request a static public IP be assigned directly to you. (This may require additional costs and will require an outage of some length during the transition). A longer but better explanation of CG-NAT can be found at the following link What is Carrier Grade NAT (draytek.co.uk)
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