I have a route set now! Thanks!
It's been awhile since I've used CodeIgniter and didn't think to configure a route to extend the controller. Makes sense now in hindsight. Below is what I have configured in my app\config\routes.php if anyone else makes the same mistake as me in the future (update the namespace).
Code:
$routes->group('', ['namespace' => 'App\Controllers'], function ($routes) {
// Login/out
$routes->get('login', 'AuthController::login', ['as' => 'login']);
$routes->post('login', 'AuthController::attemptLogin');
$routes->get('logout', 'AuthController::logout');
// Registration
$routes->get('register', 'AuthController::register', ['as' => 'register']);
$routes->post('register', 'AuthController::attemptRegister');
// Activation
$routes->get('activate-account', 'AuthController::activateAccount', ['as' => 'activate-account']);
$routes->get('resend-activate-account', 'AuthController::resendActivateAccount', ['as' => 'resend-activate-account']);
// Forgot/Resets
$routes->get('forgot', 'AuthController::forgotPassword', ['as' => 'forgot']);
$routes->post('forgot', 'AuthController::attemptForgot');
$routes->get('reset-password', 'AuthController::resetPassword', ['as' => 'reset-password']);
$routes->post('reset-password', 'AuthController::attemptReset');
});