You have properly set up one of the DNS configurations, Port Forwarding & Firewall Configuration and DNS Rebind Protection and still your NextBox is not available / reachable.
This is in particular important for Fritz!Box owners and/or DS-Lite internet connections.
First test your (sub-)domain using a 3rd party service like IPv6 Test to test your NextBox’ IPv6 reachability. If this fails this is another indicator that your IPv6 settings are not correct.
Some routers might have a bad default configuration for IPv6. Therefore please
ensure that your router is configured to pass IPv6 addresses to your network
IA_NA), if this not done, no IPv6 connection can be established
to your NextBox from the outside, thus your NextBox is not reachable.
On a recent Fritz!Box you will find this configuration inside: “Home Network -> “Network” -> “Network Settings” -> “IPv6 Configuration” Inside this view inside the section: “DHCPv6 Server in the Home Network” activate: “Enable DHCPv6 server in the FRITZ!Box for the home network:” and below this the last radio button has to be active: “Assign DNS server, prefix (IA_PD) and IPv6 address (IA_NA)” instead of the default, which is the 1st radio button.