How do I link data in a Numbers' cell to a Text box in Pages?

You don't. Apple's applications are not clones of Microsoft products (or LibreOffice) and do not provide inter-application data linking.

You would select the Numbers table data, provided it would fit legibly in portrait mode in Pages, and then paste the numbers data into the first (single-clicked) table cell of a Pages table. Or you export that Numbers data to a PDF, drop that into Pages as a document object, and then rotate it to fit on the portrait page.