Autotiles Helper by @Homunculus help


I could use some help with the Autotiles Helper asset by @Homunculus marketplace asset.
I am finding difficulty implementing it in my own project.
What I can't figure out is how to make it assign sprites to the instances already in the room, though it seemed simple enough at first.
I tried keeping only the parts of the code that I thought was needed (no luck) and then I tried putting all the objects in the room.
Shouldn't adding autotile_get_index and update_adjacent_tiles in the obj_tile code be enough for it to work?
It doesn't though. sprite _index returns -1.
I would greatly appreciate if anyone could help.
I am on 2.3 and (obviously) not very experienced.