Can’t get Fixed Header to work

I’ve been trying to get the fixed header to work on my spreadsheet.

Here’s my current shortcode

[gdoc key="https://docs.google.com/spreadsheets/d/1Y9qLdfdJkEMuq_wk0yu9FIh8YURuI1SsQFbKHpv-BGg/edit?usp=sharing" datatables_page_length="1000" csv_headers="1" header_row="1" class="FixedHeader-top"][/gdoc]

Currently the entire sheet is shown, and the scroll is within the page. Do I somehow need to constrain the sheet to a limited space within the page, so that the scroll is within that (and then the header would work)? And if so, how?

Thanks

The page I need help with: [log in to see the link]