Good morning, Flytothesky.
(1) Hopefully, someone who lives where you are currently living will
answer you with a reliable answer.
(2) Here in the United States, when a high school teacher is absent, we
call his/her replacement a "substitute teacher."
If we want to be super respectful, we call him/her a "guest teacher."
(3) You are at the university. Right? Here in the States, we usually refer
to the "professor." I notice that you use the term "lecturer."
(a) I googled and found many references to "substitute professor"
and "substitute lecturer."
(4) If no one else answers with a reliable answer, you might want to
search the Web -- concentrating on the country in which you live.
As you know, English differs from country to country. I think (not sure!)
that a substitute high school teacher in the United Kingdom is called a
a "supply teacher," a term that would really confuse an American!
Have a nice day!
Student or Learner