If you find that you cannot access or view your Synology NAS under networks in Windows 10, I advise to:
Check your Firewall software
More often that not, there is an issue with your security software and it has blocked your network. You can always try uninstalling your security software, then trying to access the network share. Remember to re-install your security and get an expert to look further at your security software.
SMB 1.0/CIFS File Sharing Support Turned Off
More commonly, especially with Windows 10, we find that SMB 1.0/CIFS File Sharing Support is turned off which also stops network sharing/access.
To check this, go to Control Panel > Programs and Features > Turn Windows Features On or Off and make sure they are ticked