Serbian railways have not shared their timetables with the rest of the world, so that's why you won't find them in the app.

You can get to Belgrade but because of engineering works, you need to get a train to Szeged, then a rail replacement bus to Kelebia, train to Subotica, another train to Novi Sad “marchalling yard” (forgot its Serbia name), a short bus ride to Novi Sad and finally a train to Belgrade.

Thank you so much!!

OR-if you have a pas with days: use the bus, much faster, less cumbersome, cheaper too. (FLIX or volan)

This line is under complete renovation and only the southern most sector in Serbije has just been finished.

ALSO note that your troubles are not over yet: as from Serbije to Macedonia and again Mac to GR NO trains over the border. You may think of a complete other routing.