It is, sounds like he was trying to port-forward for IPv4, but can't as his ISP uses a CGN or 464XLAT or something to provide IPv4 access - so port forwarding isn't an option.
All that needs to be done is add the appropriate firewall rules (probably on the router and computer) to allow traffic for that port and it'll be accessible from the outside over IPv6.
All that needs to be done is add the appropriate firewall rules (probably on the router and computer) to allow traffic for that port and it'll be accessible from the outside over IPv6.