Problem with distributed query in procedure to populate data warehouse (Oracle)

I have a problem with a procedure that I will use to populate a table in the data warehouse. I'll try to exemplify.

I have three types defined as follows:

create or replace type room_t as object(
    ID INTEGER,
    n_seats INTEGER,
    cinema ref cinema_t
) not instantiable not final;
/

create or replace type film_screening_t as object (
    screen_date TIMESTAMP,
    room ref room_t,
);
/


create or replace type ticket_t as object(
    film ref film_screening_t,
    purchase_date DATE,
    price FLOAT,
    n_ticket INTEGER
);
/

And the associated tables:

create table rooms of room_t(
    ID primary key
);
/

create table film_screenings of film_screening_t(
  room NOT NULL
);

create table tickets of ticket_t(
    n_ticket primary key,
    film NOT NULL
)
/

I also created a database link in another database that I called:

op_db_link

When I use this database link to get the id of the rooms like in the query below, everything is ok:

select deref(deref(film).room).id from tickets@op_db_link;

but when I use it in a procedure, I obtain only null values. The procedure is:

create or replace
PROCEDURE prova_procedure AS
room_id integer;

cursor c is 
select deref(deref(film).room).id from tickets@op_db_link;

BEGIN
  open c;
  loop
    fetch c into room_id;
    dbms_output.put_line(user_id);
    exit when c%notfound;
  end loop;
end;

How can I solve this problem? I need to solve this problem in order to create an ETL procedure to populate the data warehouse