If devices are in NTP sync mode, the client needs to supply the address of a NTP server which will provide the time to the device (see the 'NTP Server' field in the video wall settings for each device).

If the client has NTP access to the internet, they could use: pool.ntp.org. Or if they have an internal server they could use that.  Master and Slave require multicast communications between the devices.  If multicast comms are not supported on their network, the NTP method might be good enough, but client needs to investigate themselves.