Run these commands on the Windows Server to open the ports for listening on a named instance of SQL Server.
Note: Remember to enable TCP in the SQL Server Configuration Manager.
netsh firewall set portopening protocol=TCP port=1433 name=SQLServerTCP mode=ENABLE scope=SUBNET profile=CURRENT
netsh firewall set portopening protocol=UDP port=1434 name=SQLServerUDP mode=ENABLE scope=SUBNET profile=CURRENT
For more help, refer to SQL Server & Windows Firewall.
