I think the issue is that when you come back from the detail page, the Product Category is missing and it is loading the default which is Rentals.
To fix that you will need to pass ProdcutCategoryID to the detail page and then update the link on the detail page to include the ProductCategoryID
To do that, add a session on the Results page to store the ProductCategoryID.
$_SESSION['ProductCateogryID'] = $_GET['ProductCategoryID'];
And then update the link on the detail page to include the id by adding the following:
link.php?ProductCategoryID=<?php echo $_SESSION['ProductCategoryID']; ?>
Let me know if you have any issues.