Posting http request after enabeling TLS/SSL

Question

I have a website using Angular for the frontend, Django for the backend and they are being served using Apache. It was working properly until I enabled TLS/SSL using letsencrypt. Since then I was still able to access the website using https, but all my http request to the backend give errors. The error message is:

Http failure response for http://backend.IP:8080/api/load_data/: 0 Unknown Error

If I call the API function from the browser like this:

http://backend.IP:8080/api/load_data/

It works well and returns the expected data from the backend, but when posting http request from the code it gives the previous error.

Here is apache configurations for the frontend.conf:

<VirtualHost *:80>
  DocumentRoot "/home/ubuntu/myproject/static/"

  # Other directives here
  DirectoryIndex index.php index.htm index.html

  <Directory "/home/ubuntu/myproject/static">
      AllowOverride All
      Require all granted
  </Directory>

  # Logs

  ErrorLog /var/log/apache2/frontend_error.log
  CustomLog /var/log/apache2/frontend_access.log combined 

  RewriteEngine on
  RewriteCond %{SERVER_NAME} =test.myproject.org
  RewriteRule ^ https://%{SERVER_NAME}%{REQUEST_URI} [END,NE,R=permanent]
</VirtualHost>

Here is the frontend-le-ssl.conf:

<IfModule mod_ssl.c>
  SSLStaplingCache shmcb:/var/run/apache2/stapling_cache(128000)
  <VirtualHost *:443>
    DocumentRoot "/home/ubuntu/myproject/static/"

    # Other directives here
    DirectoryIndex index.php index.htm index.html

    <Directory "/home/ubuntu/myproject/static">
      AllowOverride All
      Require all granted
    </Directory>

   # Logs

   ErrorLog /var/log/apache2/frontend_error.log
   CustomLog /var/log/apache2/frontend_access.log combined 


   ServerName test.myproject.org
   SSLCertificateFile /etc/letsencrypt/live/test.myproject.org/fullchain.pem
   SSLCertificateKeyFile /etc/letsencrypt/live/test.myproject.org/privkey.pem
   Include /etc/letsencrypt/options-ssl-apache.conf
   Header always set Strict-Transport-Security "max-age=31536000"
   SSLUseStapling on
   Header always set Content-Security-Policy upgrade-insecure-requests
 </VirtualHost>
</IfModule>

Here is the 000-default.conf:

<VirtualHost *:8080>


  ServerAdmin webmaster@localhost
  DocumentRoot /var/www/html

  #LogLevel info ssl:warn

  ErrorLog ${APACHE_LOG_DIR}/error.log
  CustomLog ${APACHE_LOG_DIR}/access.log combined


  Alias /static /home/ubuntu/myproject/static
      <Directory /home/ubuntu/myproject/static>
          Require all granted
      </Directory>

      <Directory /home/ubuntu/myproject/myproject >
            <Files wsgi.py>
             Require all granted
            </Files>
      </Directory>

  WSGIDaemonProcess myproject python-home=/home/ubuntu/myproject/venv python-path=/home/ubuntu/myproject
  WSGIProcessGroup myproject
  WSGIScriptAlias / /home/ubuntu/myproject/myproject/wsgi.py


</VirtualHost>

Angular environment.prod.ts:

export const environment = {
    appVersion: require('../../package.json').version,
    production: true,
    apiURL: 'http://backend.IP:8080/api/',
    mediaURL: 'http://backend.IP:8080',
};

Angular proxy.conf.json:

{
  "/api": {
    "target": "http://backend.IP:80",
    "secure": false
  },

  "/media": {
  "target": "http://backend.IP:80",
  "secure": false
  }
}

Backend settings.py:

SESSION_COOKIE_SECURE=True
SESSION_COOKIE_HTTPONLY=True
SECURE_PROXY_SSL_HEADER = ('HTTP_X_FORWARDED_PROTOCOL', 'https')

ALLOWED_HOSTS = ['backend.IP',
             'http://backend.IP',
             'http://backend.IP:8080',
             'https://backend.IP',
             'https://backend.IP:8080',
             '127.0.0.1',
             'http://www.mproject.org/',
             'https://www.mproject.org/',
             'mproject.org/']

CORS_ORIGIN_WHITELIST = (
'https://localhost:4200',
'https://localhost:8000',
'https://localhost:8080',
'https://backend.IP:8080',
'http://backend.IP:8080',
'https://backend.IP:80',
'http://backend.IP:80',
)
CORS_ALLOW_CREDENTIALS = True

In the wsgi.py:

os.environ['HTTPS'] = "on"

I am completely new to these stuffs, and I followed several tutorials to reach this point but I am still missing something to allow the http requests after the setting the STL/SSL up.