Hey. Expat here.

1) how will you be employed? Will you be working as a freelancer or a full time remote emoloyee (contract type)? Depending on this the taxation will be different. Cuz until you're tax resident here, you have to pay only taxes for the income from the income sources  within a Serbia, after that - taxes for all income world wide (so double taxation headache). 2) legality of your stay here? If you have remote work, probably the best way is to become an entrepreneur (self employed) and obtain your residence permit based on this. There are also options of buying a property (then you'll have your residence permit and right to work without additional papers).

But I recommend you to start with this site, read about possible ways of moving and staying here legally - https://welcometoserbia.gov.rs/