ntwdblib.dll : How to configure it to connect over tcpip ? (SQL Server, legacy)
Try &lt;server name&gt;, &lt;port that instance is listening on&gt; as explained here.

In most cases, a named instance is not going to be listening on port 1433. In order to connect over TCP/IP, you need to explicitly specify the port number. You can find this in SQL Server Configuration Manager - under SQL Server Network Configuration, Protocols for &lt;Instance Name&gt;, right-click TCP/IP and hit Properties. The port number is near the bottom under IPAll:


  


In my case, I'm using dynamic ports, which means a different port could be assigned next time the service starts. You can force a specific, static port in the box below - hit Apply, then OK, then restart the service. You can try to use 1433 here, which might work if this is the only instance on the machine, but I haven't tried to do this for app compatibility reasons - I've always tried to make changes to the app configuration rather than SQL Server.

Then change your app to use &lt;server name&gt;, &lt;port number&gt;.