For reference if anyone comes across the same issue.

I’ve been trying to install QPKG on the device (to install a Dropbox client on it) but found a lot of obsolete information.

There is a post on a qnap forum here (https://forum.qnap.com/viewtopic.php?t=99196) that hints at an alternative option called Entware-ng.

To install it, download it from the previous link.

Then in your QNAP management page, go to the App Center, click on the cog wheel on the top right of the App Center to manually install a package.

Click Browse to locate the downloaded package, and Install.