I am very slow to upgrade.. as Apple never adds functionality.. they always remove it.
But your version of airport utility in Mojave should still have all the same screens.. albeit in terminal black instead.
Open the airport utility .. then click on the icon of your airport. A summary screen will appear with edit button.

Click the edit... and you should then get the whole works.

Disk tab I showed in earlier post is end one.
To backup the laptops and computers of a family of six, which of their products do you recommend?
Best you can afford.. the firmware is pretty much common across the whole range. For people starting out you might want to buy just a 2 disk cheap end model.. and later on you can upgrade to a 4 disk model..
Or you can buy a 4 disk model.. very much recommend more disks as it is more efficient with data redundancy.
That will be an expensive unit when you add up what 4 x 4TB (say) drives cost.. but you can start with smaller number of drives if you like and add more later.
Do not ignore backup though.. if you store files on the NAS then redundancy does not equal backup and you also should buy a large USB drive.. 8TB.. to do backup.
To justify the cost you really need to use it as much more than backup though.. you should be running it as main file server, media, music videos etc.
Also, do any of the products also interface with cellphones, which are more than ever a hub of work and activity?
Cellphones (if you are talking iPhones) do NOT work particularly well on local networks. Apple is trying to get you to use cloud storage. To even access files on the NAS you need an app like file browser. Apple restricts a lot of functions to iCloud.
Synology does attempt to replicate some of the functionality.
https://www.synology.com/en-global/knowledgebase/Mobile/help/DScloud/iOS_iPhone
Expect Apple to make future changes designed to break this.