There are a number of reasons why you might not be able to connect to a remote SQL Server. Given the error message you're receiving then it may be that you're just not specifying the instance name in the connection dialog.
I'm assuming you're using SQL Server Management Studio to try to connect. If the IP Address is correct then adding "\Name of instance on server" following the IP Address should give you a connection, e.g. 192.168.1.1\TestInstance.
A test I've occasionally found useful is to try to open a telnet session to the SQL Server port on the server. If you know that SQL Server is running on that IP Address then this can give some indication if there is a network or firewall issue:
C:\>telnet 192.168.1.1 1433
where the IP address is for your server and 1433 is the default port that SQL Server listens on for connections. If the command window goes blank then you have established a connection with SQL Server. Use CTRL+] followed by quit to terminate the telnet session.