In the modern age of technology, it is an increasingly common situation where a worker has an employment contract with a domestic employer in Serbia, but performs work from another country. This carries serious tax consequences.
The general rule of international taxation is: Income from non-self-employment is taxed in the country where the work is PHYSICALLY performed.
This means that your tax liability does not follow the seat of your employer, but follows your physical stay. If you are physically in another country while working, that country has the right to tax your work!
The greatest risk of working from abroad falls on your employer. That foreign country could declare your work a "Permanent Establishment" of your domestic company, forcing the company to pay corporate tax in that country.
For temporary "posted work," the A1 certificate system was developed to ensure that you continue to pay contributions in your home country while you are temporarily (up to 24 months) abroad.
A key international rule is 183 days: you will be considered a tax resident of the country where you have physically spent more than 183 days in one calendar year.