You can activate ssh inside the “system settings” (left side navigation, might need scrolling down). ssh is a administration feature, means enabling it for other users despite the admin (you) is neither intended nor useful for a nextcloud server. Although, if you really need this you are free to do this by yourself using your ssh access.
It’s actually there: “System Settings” → Last Section: “Reboot” & “Power off” (Could it be that you use a small resolution and you do not see the “System Settings” inside the NextBox App on the left side, then please scroll the navigation down. This is not nice, but kind of a “Nextcloud feature” we currently have no solution for.
As to be found in a very recent post here: [NextBox] Aktualisierung auf Nextcloud 21.0.4 verfügbar, we’d love to update Nextcloud, but there is a bug in Nextcloud (Fresh installation redirects to http://index.php/apps/dashboard/ (host missing) · Issue #27575 · nextcloud/server · GitHub) which is simply blocking this update. And if you check the FAQ, you will find information on how the updates happen: Nextcloud FAQ - Nitrokey Documentation (3rd point)
Can you be more precise here please? The main link for the documentation (on the Overview page) of the NextBox App leads to: NextBox - Nitrokey Documentation which is clearly working. Which other “several links from the webgui” are existing or do not work? If this is the case, we’ll happily fix that.