That sounds very like an issue I had at the beginning with my Mirror - the root cause was that my router was too old and slow to give a good enough connection for Quickview and Smartware to be able to reliably see the NAS.
A simple way to test if everything is OK with the device itself and your computer set-up (firewalls, AV etc) is OK is to plug your Mirror directly into the Ethernet port on your computer with its supplied cable. If all is well then it should appear under your Network section (I’m presuming here you’re under Windows - if not look in the equivalent section) and also be visible to Smartware and Quickview.
If it is visible then you may need to review your network set-up (the router and/or the network card(s) on your PC) to something a bit quicker. In my own case the NAS was always fully visible to an old desktop machine which was LAN-cable connected to the router, intermittently to one laptop by wifi and almost never to a second netbook connected by wifi. But when I updated my router to a new one (a Netgear D6200 in my case) all three can now always see the NAS without issue.
If it’s not visible by direct connection then you either have a local software issue (most likely a firewall configuration problem, or perhaps an AV issue) or a hardware problem. Although from what you report of it originally working I would suspect it will be visible like mine was when cable-connected directly.
Editted to add - the thread about my original problem is on the link below: