When using OCR Synching on a network with a firewall, you must enable the Bonjour service that apple uses.
According to this document:
The following ports are used by Bonjour:
You should be able to verify that you are seeing traffic on both sides of network. There are some apps that will help see the traffic.
You should see services being advertised with names that start with “_OnCue”, such as
The other On Cue specific ports that need to be open:
3700 TCP - sending transmitter page requests
3701 UDP - listening for transmitter heartbeat
8888 TCP - Sync push/pull