...
You should see an output like {"href":"./","http://letswifi.app/api#2": if the webserver is running OK internally. You should also be able to reach your server with a browser. Next, try to check the metadata with https://YOUR-ADDRESS/simplesamlphp/module.php/saml/sp/metadata.php/default-sp?output=xhtml. At this point, please obtain and install a server certificate for your web server. The configuration file is named found at /etc/apache2/sites-enabled/letswifi-portal.conf.
At this point you may find out that the SimpleSAMLphp version that your OS comes with is too old. Please download a new one and install it. Copy the apache.conf of the newly installed SimpleSAMLphp to the /config - directory and take it into use at /etc/apache2/conf-available with
...