Repair Dahua VTO-VTH Link

My VTO33110 door station refuses to discover the VTH indoor monitor, and inside the DMSS app the VTH is permanently tagged “offline.” Both devices sit on the same wired network, yet they still will not register with one another... (Budget: $10 - $30 USD, Jobs: Asterisk PBX, Linux, Network Administration, SIP, Software Architecture, Technical Support, Troubleshooting, VoIP)