Maybe I have misunderstood how this works, but when I have created the Artillery bombardment Remote and selected it, my artillery train does not go into manual mode with extended range. Its just the standard auto-range which is a lot less range than the manual mode.

So I have no idea how to actually fire it when the target is far off :)

Yeah, unfortunately I think the display change to show the larger turret radius on the map is specific to the vanilla artillery remote - I think I'll need to ask for a feature to be added to the base game to allow for showing the extended range when holding modded items.