You are right that the connection URI should be in that format
The default port for Oracle database is indeed 1521. Based on the error:
*Unable to establish connection: IO Error: The Network Adapter could not establish the connection*, it could mean that the host or the database you are connecting to may be incorrect.
You could try to replace the host parameter with the IP address of the server, as sometimes it could mean that the DNS is not resolving the hostname to the server's IP, so setting IP address would overcome this.
You could also try to telnet to the IP address and port 1521 to check if its accessible.
Hope this helps!