There must be a process on your device located in the following path : C:\Windows\System32\ebootpinger.exe.
And, the WindowsIoTCoreWatcher is supposed to listen for UDP broadcasts sent by this process every five seconds. Unfortunately, it's been reported common for this process to die or stop broadcasting. Most likely when your device hasn't been rebooted for a long time.
But, you can restart it with this PowerShell command (e.g. in a remote interactive session):