6. Is there a ban on donations from Trade Unions to candidates?

"It is prohibited to finance a political entity by foreign states; foreign natural or legal persons, except international political associations; anonymous donors, public institutions, public enterprises, companies and entrepreneurs engaged in services of general interest, institutions and companies with state capital share, other organizations discharging administrative authority and individuals who are prohibited by law to be members of political parties; trade unions, associations and other non-profit organizations, churches and religious communities; gaming industry; importers, exporters and manufacturers of excise goods, legal entities and entrepreneurs with due, and unsettled, public revenue obligations, unless set forth otherwise by this Law.
Donations from international political associations may not be in money.
Financing of political entity by a natural or legal person engaged in activities of general interest pursuant to contract with organs of the Republic of Serbia, autonomous province and local self- government units and public services founded by them is prohibited throughout the validity of such contract and for a period of two years subsequent to termination of contractual relations.
Acquisition of shares or stock in a legal person by a political entity is prohibited. Financing of a political entity by an endowment or foundation is prohibited."  
Source: Article 12  Law on financing political activities, 2022.

There is a ban.