Not quite sure what the root cause is...
The following is from the vCSHB documentation.
Channel Network connection(s) configured with the following:
- Static IP address in a different subnet than the Principal (Public) network with a different IP address than the Secondary server channel NIC
- Correct network mask
- No Gateway IP address
- No DNS server address
- NetBIOS enabled (disabled during the installation process)
Hence, if you configure non-private IP with Gateway, the servers will be able to talk to each other but Channel Network won't work as stated above.