For Set Value action to work, the property should be either associated with editable control or hidden input in UI. To achieve your requirement you can do the following,
Move the DisplayOrder property to a separate dynamic layout / section and configure it with refresh when.Level property changes.
Configure the .Level property with Post Value and Run Activity action. In this activity you can set the value of DisplayOrder property as well.
Now whenever Level is updated, the value will be posted to server, corresponding activity will be executed and DisplayOrder property will be recalculated. Due to refresh when config, the dynamic layout containing DisplayOrder will refresh itself and thereby getting the latest value.