Thank you for your reply.
Main page where both categories are displayed: index
{exp:channel:entries channel="portfolio" orderby="date" limit="50" }
<div class="box col">
<a href="http://{title_permalink=webpage/details}class=details"><span class="fade"></span></a>
{thumb}
</div>
{/exp:channel:entries}
When you click on one of the items it will take you to: details
{exp:channel:entries channel="portfolio" orderby="date" limit="1" sort="desc" }
<h1>{title}</h1>
{description}
<div class="tags">{tags}</div>
{if link != ""}<a href="http://{link}%22class=%22visit" class="visit" target="_blank" rel="noopener"></a>{/if}
</div><!-- /topcontent -->
<div class="content">{screenshoots}</div>
{/exp:channel:entries}
and next/prev entry in: details
{exp:channel:next_entry}
<a href="http://{path=webpage/details}class=next"></a>
{/exp:channel:next_entry}
{exp:channel:prev_entry}
<a href="http://{path=webpage/details}class=back"></a>
{/exp:channel:prev_entry}