Ensure that your client is unmodified.
If you're receiving the error message "Unable to connect," then you most likely are just using an incorrect password when attempting to access the gameserver with your account.
You can retrieve your password here:
https://en.nostalrius.org/passwordIf that is not your issue, then changing the IP your realmlist is connecting you to from "Login.nostalrius.org" to "Login2.nostalrius.org" might help.