As I’ve ran into this problem ,  here’s a simple and quick summary of the actions you need to take to enable it

1) Enable the TCP/IP protocol using the Surface Area Configuration Utility
2) Make sure the TCP/IP protocol is enabled in the SQL Server Configuration Utility
3)  Make sure the SQL Server browser is started. Note this step is optional. It is possible to set the SQL Server instance to use a fixed IP address – but this is non-standard for named instances.
4) Make sure SQL Server and SQL Server Browser are exempted by the firewall on the server machine. This is done by putting sqlservr.exe and sqlbrowser.exe as an exception in the windows firewall.
5) In order to get things to work. You might need to completely reboot the server machine after making the changes. There have been reports that starting and stopping the SQL Server and Browser software is not enough.