Failing HTTPS/SSL Connection when trying to connect to a server on an Andoid-Phone

I'm currently working on a university project where I'm setting up an Ubuntu Server to correspond with an existing app. The app should be able to send data and pictures to my server via a post-request. The server is supposed to process the data and save it.

Everything works fine when I'm running it in Ionic on my computer. However, as soon as I start testing it on a smartphone, I encounter this error:I'm unsing a Google Pixel with Andoid Version 14

https://localhost/polyfills.826f909f93350bf7.js - Line 1 - Message: Mixed Content: The page at 'https://localhost/form' was loaded over HTTPS, but requested an insecure resource 'http://###IP-Address###:8080/Server.php'. This request has been blocked; the content must be served over HTTPS.

When I change the address of my server to https://..., I get the following error:

[ERROR:ssl_client_socket_impl.cc(970)] handshake failed; returned -1, SSL error code 1, net_error -200

So I tried to use a self-generated SSL certificate. Since all of this is just for demonstration purposes and the server is never going to be accessible outside the university network.

Here's what I've done so far:

1. Configured my server like this:

   /etc/apache2/sites-enabled/000-default.conf:

   <VirtualHost *:8080>    ServerAdmin webmaster@localhost    DocumentRoot /var/www/html    ErrorLog ${APACHE_LOG_DIR}/error.log    CustomLog ${APACHE_LOG_DIR}/access.log combined    # SSL Configuration    SSLEngine on    SSLCertificateFile /etc/ssl/certs/certificate_ssl.pem    SSLCertificateKeyFile /etc/ssl/private/key.pem<Directory /var/www/html>        Options Indexes FollowSymLinks        AllowOverride None        Require all granted</Directory></VirtualHost>

   /etc/apache2/sites-available/default-ssl.conf:

   SSLCertificateFile   /etc/ssl/certs/certificate_ssl.pemSSLCertificateKeyFile /etc/ssl/private/key.pem

2. In the app, I made the following changes:

   Created this file android\app\src\main\res\xml\network_security_config.xml, containing this:

   <?xml version="1.0" encoding="utf-8"?><network-security-config><debug-overrides><trust-anchors><certificates src="@raw/debug_cas"/></trust-anchors></debug-overrides></network-security-config>

   I put my certificate here: android\app\src\main\res\raw\debug_cas.pem

3. My AndroidManifest.xml looks like this:

   <?xml version="1.0" encoding="utf-8"?><manifest xmlns:android="http://schemas.android.com/apk/res/android"><application        android:allowBackup="true"        android:icon="@mipmap/ic_launcher"        android:label="@string/app_name"        android:roundIcon="@mipmap/ic_launcher_round"        android:supportsRtl="true"        android:networkSecurityConfig="@xml/network_security_config"        android:theme="@style/AppTheme"><activity            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|smallestScreenSize|screenLayout|uiMode"            android:name=".MainActivity"            android:label="@string/title_activity_main"            android:theme="@style/AppTheme.NoActionBarLaunch"            android:launchMode="singleTask"            android:exported="true"><intent-filter><action android:name="android.intent.action.MAIN" /><category android:name="android.intent.category.LAUNCHER" /></intent-filter></activity><provider            android:name="androidx.core.content.FileProvider"            android:authorities="${applicationId}.fileprovider"            android:exported="false"            android:grantUriPermissions="true"><meta-data                android:name="android.support.FILE_PROVIDER_PATHS"                android:resource="@xml/file_paths"></meta-data></provider></application><!-- Permissions --><uses-permission android:name="android.permission.INTERNET" /></manifest>