You are not setting the item by reference.
The datatable will also reset upon the end of gameplay. It does not have a built in load / save function. You would have to use the datatable function convert struct to JSON string and save it as a field in your savegame.
Though loading would probably need c++
Though I doubt there is need for a datatable for the characters inventory. You can just save out the TArray of structs to a save file and reload it on game load de-serializing it and populating the array again.