Has anybody had experience interfacing Schneider Twido RS485 PLC with 900HP Modems?

Trying to replace RS485 cable with RF modem. The interface with the Schneider PLC seems flaky, works occasionally then not at all for a while. Running in 2 wire mode. Have tried with and without SG connection. Schneider advise that the RS485 on Twido is non isolated. Termination turned on is the only way it seems to work. Voltage levels maybe? Using a Digi 900HP RS485 module.

Thank you