im wondering if you setup passthrough correctly on your Hitron CGN-3
Can you confirm if your ISP is using carrier-grade NAT? This sounds like a case of that happening.
If that's the case you can't portforward and you need to tell your ISP to give you a public IP address so you can portforward.
Carrier-grade NAT is a way used by ISPs to deal with the exhaustion of IPv4 addresses.