submit button not working correctly
Your form function is called redirect_custom_reg_user_register_form(), so your submit function needs to be called redirect_custom_reg_user_register_form_submit().

If you want to keep the same submit function name that you have currently, you need to add it manually in the form function as Drupal only looks for a function called FORM_FUNCTION_NAME_submit() by default:

$form['#submit'][] = 'redirect_custom_reg_submit';