Are you sure your servername is 'SQL' and the instance name is 'MSSQLSERVER'?
If you can connect using the same settings in SSMS it should work without problems if the 'db_username' user have the sufficient access rights?

Your connection window should like this : 

The logs are stored here : Portals\_default\Logs and are of type 'resources' to avoid being downloaded by mistake.

Thanks.

The server is technically sql2012 (I shortened it for brevity).  

I do not need the instance on the other machine (I can join the machine from SSMS using just the server, user name and password).  

I will check the logs later.  Hopefully there is a more verbose reason for the failure.

The instance name MSSQLSERVER is normally the default instance of SQL Server - therefore it should not be necessary to add it to the connection string ("not necessary" should mean "would not work with it").

Another problem could be a firewall between the web server and the SQL Server (if they are not on the same machine). Please check if you get an answer from port 1433 (or whatever your instance uses).

If you use an SQL Server Login (instead of integrated security, which is highly recommended) you have to activate Mixed authentication mode in SQL Server.

Happy DNNing!
Michael

Please check out the TCP port open in SQL time the TCP service is off you can't access your server also please share the MSSQL database login screenshot along with the installation screenshot