Thanks for the reply Alex.
This modem is supplied here in Australia by the Unitronics dealer, it is working fine.
I just have the same problem as stated by the OP, of it only working for a short period of time (1-2 days) before needing to be reset.
I was hoping for a fix to that particular issue, not sure if its the modem or PLC.
My biggest problem is fault finding, as I can't look at code and use modem at the same time on an M90.