Where did you see that error message? With NC?
Do you mean your router is not able to handle NAT Loopback? If not, you would have a challenge to access your DDNS from inside your local network. From external, there would be no issue to access NC using DDNS + https. However from internal, some other workarounds needed to manipulate DNS request inside your network.
Depending on actual situation we could move ahead step by step to activate HTTPS, DDNS and HTTP/2