Hi,
The problem comes from the CSS of your template which hides these buttons:
body .hikashop_product_page span.hikashop_previous_product, body .hikashop_product_page span.hikashop_next_product {
display: none;
}
in the file templates/r_rnshop/custom/css-compiled/hydrogen_17.css
I would recommend contacting the support of your template provider because that's not right. They shouldn't hide them. They can change the CSS of these if they want in their template, but hidding them in CSS is not a good solution since then people won't see them when they activate the display of these buttons like you did.
