Synology NAS not showing in File Explore
I was having this problem often. It is related to the Local Master Browser setting. If you are operating a NAS Drive, it is best to set the NAS Drive as the Local Master Browser. If you are using a Synology NAS Drive the setting is located in Control Panel/File Services/Advanced Settings. Enable Local Master Browser.
The problem arises if you operate a number of computers on a network. The first computer that switches on becomes the Local Master Browser by default. When that computer switches off, all devices on the network will disappear. If you then switch it on the devices on the network will not reappear, unless you switch off/on all the other devices.
The best setting is to set the NAS Drive as Local Master Browser and leave it on. It will go into idle when the computers are switched off, but as it is always on, all devices will appear on your network when you switch any computer on the network on.