Diagnose Offline Dahua VTH Units

I have between two and five Dahua indoor monitors (VTH series) that now show as “offline” in the system interface. I only noticed the issue recently, so I’m not sure whether they all failed together or... (Budget: $10 - $30 USD, Jobs: Cisco, Linux, Network Administration, Network Security, Technical Support, Troubleshooting, VPN, Wireless)