By default the Total theme displays an add to cart button when hovering over the product image for product entries. If you wanted to display a static button mobile only it's possible with a little code. The following code re-adds the static button and hides it on all devices over 1024px
// Re-add the static add to cart button.
function my_add_product_mobile_add_to_cart() {
add_action( 'woocommerce_after_shop_loop_item', 'my_custom_product_add_to_cart', 10 );
}
add_action( 'init', 'my_add_product_mobile_add_to_cart' );
// Custom function for displaying the add to cart button mobile only.
function my_custom_product_add_to_cart() {
echo '<div class="wpex-lg-hidden">';
woocommerce_template_loop_add_to_cart();
echo '</div>';
}