Once in Niš, you can rely on different types of public transport:
BUS: http://www.busevi.com/red-voznje/categoryevents/7-gradski-prevoz-ni
https://getbybus.com/en/blog/buses-nis/
SOME TAXI COMPANIES:
Flix: https://flix.rs/en/
Bros Taxi: http://www.brostaxi.rs/
Cool Taxi: http://www.cooltaxi.rs/
Eko Taxi: https://ekotaxi.rs/
More information finde here: https://www.ni.ac.rs/en/student-info/living-in-nis/transport-in-nis