EF Core context open connection after a using statement closes it

In a .NET core web api app I'm using ef core in a function that i have to run some raw sql then when I try to use the context after the raw sql it fails.

the context is passed from the from controller to logic files(actual business code) is a private readonly I've tried opening the connection again but that fails.

DbConnection conn = _context.Database.GetDbConnection();
using (conn)
{
   await conn.OpenAsync();
   using (DbCommand cmd = conn.CreateCommand())
   {
      //Raw SQL                
   }
}
Table1 t1 = await _context.Table1
      .Where(i => i.id == 1)
      .SingleOrDefaultAsync();

Everything works fine until the app gets to the Table1 t1 line then i get the error

The error I get is

InvalidOperationException: The ConnectionString property has not been initialized. System.Data.SqlClient.SqlConnection.PermissionDemand()

Edit

I'm asking is there a best practice way to run raw SQL that returns data that is not mapped to an entity then continue to use EF Core. Also sometimes my raw SQL is only returning a string or decimal value

Edit #2

Thanks for the help I'll be using the suggestion made by Ivan Stoev

Updated working Code

DbConnection conn = _context.Database.GetDbConnection();
await context.Database.OpenConnectionAsync();
using (DbCommand cmd = conn.CreateCommand())
{
   //Raw SQL                
}
context.Database.CloseConnection();
Table1 t1 = await _context.Table1
      .Where(i => i.id == 1)
      .SingleOrDefaultAsync();