404 when accessing a function in a controller

Hi,
This is my first iteration of Codeigniter and I'm having trouble calling a function inside a controller.
I've used this tutorial more or less verbatim to set up: https://www.positronx.io/codeigniter-aut...-tutorial/
I have modified the 'welcome view' (https://example.com) with 2 buttons, 'Access' and 'Register'. 
I have 2 Controllers, 

Code:

SignupController.php & SigninController.php

In the signup controller, there are 2 public functions:

PHP Code:

public function index() 


which calls the 'form' helper and invokes the 'signup' view, and

PHP Code:

public function store() 


which verifies the input data and inserts it to the database.
The signup view contains the form html:

Code:

<form action="<?php echo base_url(); ?>/SignupController/store" method="post">

When I click the 'Register' button on the welcome page, this correctly calls the signup page: example.com/signup
But when I click the 'Submit' button of the form, example.com/SignupController/store, I get 404 File not found.
Can anyone suggest where I'm going wrong?
The Routes.php file has

PHP Code:

$routes->get('/signup', 'SignupController::index'); 


and I have tried to add

PHP Code:

$routes->get('/signup', 'SignupController::store'); 


, but it makes no difference.