In my case (D8), I just had to implement the RedirectCommand in my AjaxSubmit :

Code of the submit button :

$form['submit_' . $this->uniqueIdentifier] = [
  '#type' => 'submit',
  '#value' => $this->t('Submit'),
  '#attributes' => [
    'class' => ['btn', 'btn-primary']
  ],
  '#ajax' => [
    'callback' => [$this, 'submitModalFormAjax'],
    'event' => 'click',
  ],
];

Code of the submit callback:

public function submitModalFormAjax(array $form, FormStateInterface $form_state) {
  $response = new AjaxResponse();

  if ($form_state->hasAnyErrors()) {
    // Do validation stuff here
    // ex: $response->addCommand(new ReplaceCommand... on error fields
  }
  
  else {
    // Do submit stuff here

    $url = Url::fromRoute('page_route');
    $command = new RedirectCommand($url->toString());
    $response->addCommand($command);
  }
  
  return $response;
}

Do not forget to declare :

use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\RedirectCommand;

In my case (D8), I just had to implement the RedirectCommand in my AjaxSubmit :

Code of the submit button :

$form['submit_' . $this->uniqueIdentifier] = [
  '#type' => 'submit',
  '#value' => $this->t('Submit'),
  '#attributes' => [
    'class' => ['btn', 'btn-primary']
  ],
  '#ajax' => [
    'callback' => [$this, 'submitModalFormAjax'],
    'event' => 'click',
  ],
];

Code of the submit callback:

public function submitModalFormAjax(array $form, FormStateInterface $form_state) {
  $response = new AjaxResponse();

  if ($form_state->hasAnyErrors()) {
    // Do validation stuff here
    // ex: $response->addCommand(new ReplaceCommand... on error fields
  }
  
  else {
    // Do submit stuff here

    $url = Url::fromRoute('page_route');
    $command = new RedirectCommand($url->toString());
    $response->addCommand($command);
  }
  
  return $response;
}

Do not forget to declare :

use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\RedirectCommand;
use Drupal\Core\Url;