CI4 where should I connect to DB?

Oops, it does not work :

You must set the database table to be used with your query.

Here is my code:

Code:

$builder = $this->table('tablename');
        $builder->orderBy('id', 'DESC');
        $builder->orderBy('timestamp', 'DESC');
        $builder->where('field', $field);
        $sqlQuery  = $builder->get();       
return $sqlQuery->getResult();

It only works with manual initialization. What am i doing wrong?