1. This is the tracking of your parcel:
China Post Register Air Mail Tracking System
Parcel No.RS807754311CN tracking result
2016-11-17 09:33:14.0 Electronic Data Received
2016-11-18 23:10:16.0 Guangzhou international parcel process center received
2016-11-18 23:43:34.0 Guangzhou parcel process center customs scan
2016-11-18 23:50:40.0 leaving Guangzhou parcel process center ，next station Guangzhou Airport
2016-11-19 06:54:49.0 Guangzhou Transit Station export security scan
2016-11-22 13:07:04.0 Guangzhou Transit Station export customs scan (domestic transit )
2016-11-27 19:02:47.0 Beijing Transit Station export security scan
2016-11-28 09:33:06.0 Beijing Transit Station export customs cleared
2016-11-28 22:25:00.0 arrive Beijing Airport
2016-11-28 22:29:14.0 leaving Beijing Airport going to next airport/seaport
2016-12-07 11:31:00.0 Beijing departure

2. Parcel to Serbia in normal condition should take 21 days to arrive. Your parcel has pass that limit. Either your parcel is lost or misplace or Serbia Post did not scan your parcel yet and still waiting into the incoming Inbound warehouse for overseas parcels.

3. May I suggest to contact your seller and tell him you have not receive your parcel. In worst case scenario open a dispute for non reception of parcel.

Thanks.