Second Dropdown value not populating After selecting the First dropdown In Multi-Page App Structure


Below is the structure of my project, [dd-> refers to dropdown], The bold ones are directories, except

|- assets
|- icon-blue.jpg
|- components
| [Contains all the functions to connect to the dB & and get values to fill the 1st dd, and based on the selected value of 1st dd, 2nd dd get populated]
|- [Ignore]
| [Ignore]
| [contains the callback to fill the 2nd dd, based on the selected value of 1st dd]
| [contains the layouts for 1st and 2nd dropdown]

The issue, the app works completely in a single app format, so there is no issue with code or imports hence i am sharing file which i believe have some issue that i am unable to figure it out.

In, I have not defined any separate pathnames or link other than ‘/’, and want the whole layout in ‘/’.

import dash
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output
from layouts import layout_base
import callbacks
from flask import send_from_directory

external_stylesheets = ['']

app = dash.Dash(__name__, external_stylesheets=external_stylesheets, url_base_pathname='/')

app.css.config.serve_locally = True
app.scripts.config.serve_locally = True
app.config.suppress_callback_exceptions = True

app.index_string = '''
<!DOCTYPE html>
        <h1>Advertiser Conversion Dashboard</h1>
            <div>Advertiser Conversion Dashboard</div>


app.layout = html.Div([
    html.Link(href='/assets/skeleton.min.css', rel='stylesheet'),
    html.Link(href='/assets/font-awesome-min.css', rel='stylesheet'),
    html.Link(href='/assets/normalize.min.css', rel='stylesheet'),
    html.Link(href='/assets/font.css', rel='stylesheet'),
    html.Link(href='/assets/jquery-3.2.1.min.js', rel='script'),
    dcc.Location(id='url', refresh=False),

# Update page
# # # # # # # # #
@app.callback(Output('page-content', 'children'),
              [Input('url', 'pathname')])
def display_page(pathname):
    if pathname == '/':
        return layout_base
        return '404'

def static_file(path):
    static_folder = os.path.join(os.getcwd(), 'assets')
    return send_from_directory(static_folder, path)

# Run the App Server
if __name__ == '__main__':
    app.run_server(debug=True, port=9002)

Can you help when i am getting wrong ?