panos21g:
Any thoughts?
The client is using the wrong hostname, or the hostname it is using has not resolved to the correct address, of the address you're using is wrong, or the port is wrong, or port forwarding hasn't been set up correctly, or there is a firewall somewhere between the client and server which is blocking the connection, or you do not have a route from your router to your Arduino, or the Arduino is not listening to the port you are trying to use on the address you are trying to use.
None of these are Arduino issues.