This article explains the causes behind an internal server reporting as unreachable via its external IP address.
Typically this problem is seen on guest SSIDs where an alternate DNS is configured to provide guest clients with the external IP address of the internal server. This can result in guest clients being unable to communicate with the internal server via the external IP address.
Trying to access an internal server via its external IP address on the same network the internal server is hosted on will typically not work as expected, unless your router supports (and you've enabled) NAT Hairpinning. NAT Hairpinning may also be called NAT Reflection or NAT Loopback.
Not all routers offer this functionality, call it by the same name or have the same configuration options. You'll need to review your router documentation or talk to the router manufacturer's support staff for more details about whether it's supported, and how to configure it.