When you failed to connect you'll see a reason in the red box that appears under the apply button.
Failed to communicate with router:
Firstly please try disable your anti-virus, adware or firewall and see if the error goes away.
If the error does not go away and you see failed try again in 5 s connection timed out in the log then you are trying to connect to a server that is down. Please untick enable click apply. Reboot the router and try connect to a different server please.