Unable to Call an On-Premise NON SAP System API via API Management

I have an .NET Web API hosted on an OnPrem server. The server is reachable via the Cloud Connector and it is also showing Connected under SCP.

However, when i am trying to create an API Provider inside API Management, under On-Premise
It is giving an error

Connection to the system was unsuccessful with HTTP response code: 503; Message : Service Unavailable

Is it even possible. If not what alternatives do we got?