I regressed the firmware to 1.14.1 but that did not help. I then regressed it to 1.10 and that seemed to make things better but the IP address was still being reported as 0.0.0.0 even though it was assigned in the WebUI. I then reinstalled 1.14.3 and the Modbus monitoring seems to work correctly now.
It's strange, but I now have a solution - but if anyone has any similar experience or can shed any light on it then I'd be interested to hear it. Note that my modem did not ship with 1.14.3 installed, so that was an upgrade that I did before I started configuring the modem.