Variable Instance for BACnet Read node


I am using "BACnet Read" node, which contains parameters to read from BACnet Device. However, one of the parameters is "Instance". I need to make this instance variable to be modified by the user (or an external system).
To do that, I have added a "Text Input" node. I changed its output to "flow" and stored it in a variable. I was able to read this variable from a separate function and also view it. However, Once I add this variable to the "Instance" parameter, nothing changes. It seems that the "BACnet Read" node does not see it at all.

Would you please help me with how to make this "Instance" variable?
Thanks in advance,