If I was to made an educated guess the Ethernet port would be to the network the cable boxes to the Cable modem/Gateway similar to the MoCA (coax) connection. There are 3 standards for networking devices in your home - Ethernet, MoCA and Powerline. Considering that boxes are IP based they need a connection back to Flow's headend and that is through the DOCSIS network aka cable modem.
Here is a test, connect the Ethernet port from the cable modem to the Hybrid IP DTA instead of the Coax. In theory that should also work.