Hi TomaszW-0873,
You can try to use the following connection string in web.config:
<add name="ConnectionStringName"
providerName="System.Data.SqlClient"
connectionString="Data Source=ServerName;Initial Catalog=DatabaseName;Integrated Security=False;User Id=userid;Password=password;MultipleActiveResultSets=True" />
For more details, please refer to SQL Server Connection Strings for ASP.NET Web Applications and SQL Server Database Connection To MVC Application which might help.
In addition, your issue is more related to MVC. I suggest you open a thread in the ASP.NET MVC forum so that people there will help you more effectively.
Best Regards,
Amelia
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