Custom User Registration Form not showing

Presuming you have your custom registration form set up to its own URL, such as /custom-register-page/ or something like it, then you can use the following code to change the register URL on the default WP login page:

add_filter( 'register_url', 'update_register_url' );
function update_register_url( $url ) {
    if( is_admin() ) {
        return $url;
    }
    return "/custom-register-page/";
}

Add this code to your template’s functions.php file, if that makes sense, or create a plugin to use this code. Make sure to update the code to use your custom register page URL.