Based on DNS tests, your nameservers aren't resolving correctly, which would explain the error you're getting.
Make sure your DNS configuration is set up correctly. It needs to be pointed to GoDaddy nameservers if you're hosting here.
As to the changes they said to make in the wp-config file, that's a little vague. You will want to talk with your developer to ensure it gets set up correctly or do some research on the net to see if you can't diagnose your issue. The information provided just isn't enough for anyone here to make an educated diagnosis of the problem.
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