You can check my topic Modbus binding: resend values to device after power down and i’m sure you will get it working as i think everything is explained there in the last post.
In the wago PLC, I use M bits to control the outputs.
That way I can control the outputs via PLC or OpenHab.
In the PLC it looks like this:
output := (modbus bit OR PLC bit)
If you have anymore questions just ask and I will try to check my configuration again and give more info.