entities – Drupal 8: Change $form attribute in callback

I have a form and a field model. In a hook alter, I add ajax callback to a function getMobileorFixed that does some taxonomy computation. Here is the hook_alter

function THEME_form_FORM_ID_alter(&$form, &$form_state) {
    $form('elements')('system_information')('model')('#ajax') = (
        'callback' => 'getMobileorFixed',
        'event' => 'change',
        'progress' => (
          'type' => 'throbber',
          'message' => t('Loading...'),

it works. and when I wrote getMobileorFixed it also computes the value:

function getMobileorFixed(array &$form, FormStateInterface $formState){
  $equipment_id = $term_obj->get('field_equipment_type')->target_id;
  $equipment_taxonomy = Drupal::entityTypeManager()->getStorage('taxonomy_term')->load($equipment_id);
  $equipment_type = $equipment_taxonomy->get('name')->value;
    //these 2 lines temporarily add the value to `equipment_type` (#value / #default_value) attributes
  $form('elements')('system_information')('equipment_type')('#value') = $equipment_type . " (" . $equipment_id . ")";
  $form('elements')('system_information')('equipment_type')('#default_value') = $equipment_type . " (" . $equipment_id . ")";
  //calls a `myAjaxCallback` in my javascript file
  $response = new AjaxResponse();
  $response->addCommand(new InvokeCommand(NULL, 'myAjaxCallback', ()));
  return $response;

However, later on submission the:


are empty. I am having trouble understanding how to permanently set values even inside callbacks. I thought perhaps passing in the reference variable array &form would change the values once the function returns, but that does not seem to be the case. Any help would be appreciated. thank you.