The expected behavior in this case: values and statuses of tags are zero when connection is broken. Does it work as expected or not?
“Unable to request because connection is not established” – probably, the device disconnects from the PC. If this is a usual LAN, you should check communication with Modbus Simulator installed on another PC. If it works normal, the problem is in the device.
I understand the software is working as designed. Is it possible for the last good value (last value when device is connected) to display. Instead of defaulting to zero.
Also is it possible to display a timestamp of the when the device was connected last?