The Docs / Examples for EmonModbusTcpInterfacer appear to be missing the information of how to set the Modbus unit ID.
From what I can find in an old forum thread you need to do:
[[[runtimesettings]]]
register = 305, 306, 314, 321, 0xA001
# unit number for each Register
nUnit = 2, 2, 2, 2, 2
This seems to have worked.