Android application unable to send http requests

I have an android application that uses Retrofit to make http requests (POST) and further evaluate incoming data.
I have the app working absolutely fine in my test phone (Samsung J5 P), it connects and sends requests as it should. But on trying operating the app simultaneously on two different phones, the app on the other phone somehow fails to send requests (retrofit calls onFailure method when a connection fails). I have enabled all permissions, yet the problem seems to happen.

Also, I have hosted the server on my desktop (localhost), and I made sure both my devices are connected to same network.

What could be causing this problem? Thanks.

Create a network config file in res/xml directory

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <domain-config cleartextTrafficPermitted="true">
        <domain includeSubdomains="true">YOUR_IP</domain>
    </domain-config>
</network-security-config>

and then add android:networkSecurityConfig="@xml/network_security_config" in your manifest's application tag

Starting with Android 9 (API level 28), cleartext support is disabled by default.

Also have a look at - https://koz.io/android-m-and-the-war-on-cleartext-traffic/

Codelabs explanation - https://codelabs.developers.google.com/codelabs/android-network-security-config/index.html

Option 1 -

First try hitting the URL with "https://" instead of "http://"

Option 2 -

Create file res/xml/network_security_config.xml -

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <domain-config cleartextTrafficPermitted="true">
        <domain includeSubdomains="true">api.example.com(to be adjusted)</domain>
    </domain-config>
</network-security-config>
AndroidManifest.xml -

<?xml version="1.0" encoding="utf-8"?>
<manifest ...>
    <uses-permission android:name="android.permission.INTERNET" />
    <application
        ...
        android:networkSecurityConfig="@xml/network_security_config"
        ...>
        ...
    </application>
</manifest>
Option 3 -

android:usesCleartextTraffic Doc

AndroidManifest.xml -

<?xml version="1.0" encoding="utf-8"?>
<manifest ...>
    <uses-permission android:name="android.permission.INTERNET" />
    <application
        ...
        android:usesCleartextTraffic="true"
        ...>
        ...
    </application>
</manifest>

 android:targetSandboxVersion can be a problem too -

According to Manifest Docs -

android:targetSandboxVersion

The target sandbox for this app to use. The higher the sandbox version number, the higher the level of security. Its default value is 1; you can also set it to 2. Setting this attribute to 2 switches the app to a different SELinux sandbox. The following restrictions apply to a level 2 sandbox:

The default value of usesCleartextTraffic in the Network Security Config is false.
Uid sharing is not permitted.
So Option 4 -

If you have android:targetSandboxVersion in <manifest> then reduce it to 1

AndroidManifest.xml -

<?xml version="1.0" encoding="utf-8"?>
<manifest android:targetSandboxVersion="1">
    <uses-permission android:name="android.permission.INTERNET" />
    ...
</manifest>