submitForm not being called when put inside a Custom Block

I have been trying to find a fix for this but I haven't been able to. I have tried to put a form inside a custom block to render the form as a component. When the form is submitted it renders the component again but it doesn't invoke the submitForm function. Please review the code and let me know if you guys understand what the problem is.

NewsletterForm.php

<?php
/**
 * @file
 * Contains \Drupal\newsletter_block\Form\WorkForm.
 */

namespace Drupal\newsletter_block\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

class newsletterForm extends FormBase {
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'newsletter_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {

    $form['newsletter_email_id'] = array(
      '#type' => 'email',
      '#title' => t('E-MAIL'),
      '#required' => TRUE,
    );

    $form['submit'] = array(
      '#type' => 'submit',
      '#value' => $this->t('SUBSCRIBE'),
    );

    return $form;
  }

  public function submitForm(array &$form, FormStateInterface $form_state) {
   
   /**
   * {@inheritdoc}
   */

    $obj = array(
        'newsletter_email_id' => $form_state->getValue('newsletter_email_id'),
    );

    $url = 'http://demo7220892.mockable.io/newsletterblock';
    
    $options = array(
        http => array(
            'header' => "Content-type: application/www-form-urlencoded",
            'method' => 'POST',
            'content' => http_build_query($obj),
        )
        );
    $context = stream_context_create($options);
    $result = file_get_contents($url, false, $context);
    if ($result === FALSE) {
        drupal_set_message(t('Error in sending data to the server. Please try again later!'), 'error');
    }
    else {
        drupal_set_message(t('Thank you for subscribing to our newsletter. We will keep you updated on our latest developments.'));
    }

    var_dump($result);
  }
}

NewsletterBlock.php

<?php
/**
 * @file
 * Contains \Drupal\newsletter_block\Plugin\Block\NewsletterBlock.
 */


namespace Drupal\newsletter_block\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Provides a 'NewsletterBlock' for the Footer.
 * 
 * @Block(
 *   id = "newsletter_block",
 *   admin_label = @Translation("Newsletter Block"),
 * )
 * 
 */

 class NewsletterBlock extends BlockBase {
    
   /**
   * {@inheritdoc}
   */

  public function build() {
    
    $form = \Drupal::formBuilder()->getForm('Drupal\newsletter_block\Form\NewsletterForm');

    return $form;
  }

}

newsletter-block.html.twig

<div class="newsletterBlock" id="newsletter_block">
    <h4 class="newsletterTitle fw-normal">Newsletter</h4>
    <form>
        <div class="form-group">
            <input class="newsletterEmail fw-normal" name="newsletter_email_id" id="newsletterEmailAddress" aria-describedby="emailHelp" placeholder="E-MAIL"><input type="submit" name="subscribe_button" id="subscribeButton" value="SUBSCRIBE"/>
        </div>
    </form>
</div>

{{ attach_library("newsletter_block/newsletter-block") }}

{{ form.form_build_id }}
{# required #}
{{ form.form_id }}
{# required #}
{{ form.form_token }}
{# required #}

newsletter_block.module

<?php

/**
 * Implements hook_theme().
 */
function newsletter_block_theme($existing, $type, $theme, $path) {
  return [
    'newsletter_block' => [
      'template' => 'newsletter-block',
      'render element' => 'form',
      ],
    ];
}

function newsletter_block_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == 'newsletter_form') {
    $form['#theme'] = 'newsletter_block';
  }
}

The form is being rendered properly but clicking the SUBSCRIBE button does not invoke the submitForm function. I know this because I tried using breakpoints through XDebug. Please help me out with this.

Thank you in advance!

You don't need a form alter hook, you can define the form template

$form['#theme'] = 'newsletter_block';

in buildForm() of the form class.

Getting the form submitted

The form template needs to render the form elements of the form build provided in the variable form and at the end you can render the rest of form so you don't need to specify all the required internal stuff:

newsletter-block.html.twig

{{ form.newsletter_email_id }}
{{ form.submit }}

{{ form|without('newsletter_email_id', 'submit') }}

Styling form elements

When form elements need specific attributes add them already in buildForm():

$form['newsletter_email_id'] = [
  '#type' => 'email',
  '#title' => t('E-MAIL'),
  '#required' => TRUE,
  '#attributes' => [
    'class' => [
      'newsletterEmail',
      'fw-normal',
    ],
    'aria-describedby' => 'emailHelp',
  ],
];