[Answered ]-Django dependant filters using json list variable contained within a foreign key

The form is only created once during rendering of the template. You would have to use JavaScript to manipulate the choice field. Either you render all possible routers and their interfaces in a script tag of your html that can be accessed by a js-file or you place an AJAX call based on your router choice to return the appropriate port choices and fill the field with the data from the response.