DAC connection error
Check the SQL Server error log to determine the remote DAC port number.  It will be contained in the message Dedicated admin connection support was established for listening remotely on port nnnn..  Verify that port is allowed through the firewall.  

You can verify TCP port connectivity with TELNET or the Powershell one-liner below, specifying the port number as the argument piped to the echo command.

nnnn | % { echo ((new-object Net.Sockets.TcpClient).Connect("YourServerName",$_)) "server listening on TCP port $_" }