With High Sierra, my NAS SmartStor NS4300N automatically appeared in the Finder as a volume. When it didn't, I could easily look up the NAS' IP address in my router and connect via SMB.
With Mac OS X 10.14.2 Mojave, the NAS is no longer mounted automactically. It appears in the Finder under "Network", but there is no chance to connect. When I click "Connect as ..." in the Network window, it takes some time then results in an error "Error when connecting to the server "nas : SmartStor NS4300N". Check the server name or the IP address and try again. With further problems contact your system administrator." (translated from German)
The NAS doesn't appear in my router, either.
The programm LanScan.app shows all devices connected to the Network, but the NAS is not listed.
What can I do to get a connection again?