Hmm thats the one I bought as well. Havn´t had time to test it though. I guess its pointless now 
I have to get the other one as well then, as I want to get rid of all those damn USB dongles which is driven me nuts on openhabian.
For now I´m only using USB RS485 for my Nilan ventialtion system (RTU modbus). Thats the one I want to get an ethernet <> serial RTU for. I thought the Waveshare would do.
As mentioned I first ordered Waveshare RS485 to Ethernet Converter, but it is intended for another use use case as @rossko57 mentioned. Then I received my USR IoT USR-TCP232-410S device, and that one works perfect together with openhab Modbus.
All my previous problems with serial modbus connection are gone now with the device, and I can enable and disable Modbus poller as I wish without problems.
@ssalonen : Thank you very much for your support.
Glad to hear that.
It seems so odd that a different USR product (and supposedly higher spec.) should give trouble.