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That it uses 192.168.1.5 still, is a sign that the basic networking functions, in hardware at least, are still 'alive'. But, having the Zyxel box not respond to NSA320 software nor to browser, means it has a serious internal problem.
In short: it's only responding at the network hardware level. Either the disk or disks have a problem or else the control circuitry that turns disk reads into network messages and responds to queries and requests from the network (from web browser or NSA320 software.)
It is of course most likely that a disk has gone bad in it. How old is the Zyxel NSA320 box in question? I ask in order to determine whether it is still covered by warranty terms.