While there are job opportunities in Serbia, predominantly in Belgrade and Novi Sad, I recommend that you either look for online teaching jobs or consider starting your own teaching business here (and apply for a business owner temporary residence permit).
If you're still looking to work for someone else, make sure to land a job before you move. It's probably a good idea to have a lawyer or solicitor review your employment contract before you sign it. Things don't always go "as expected" in Serbia.
Salaries are modest compared to Western Europe, however they are sufficient for a comfortable life here. A TEFL or CELTA certification is a major asset for finding positions, especially in larger cities like Belgrade.
Read more: https://teast.co/teach-english-serbia