The shrine of Isis needs to be cleared so that rituals can take place there, and we can be lucky to find another plate piece here.
You must dig all the tiles and for the plate piece, fix the pipes first, then water the trees by turning the valves. Watering 3 trees will open the 3 gates and you will be able to get the plate piece.
If you need help with putting the pipes on the correct place, watch this video
0:53 for the bottom right pipes.
1:43 for the upper pipes.
2:37 for the bottom left pipes.
Finally, talk to the priest of Isis in town to complete the quest.