FaceTime region: Serbia

Hello. I buy iPad 9 (WIFI) for my mom (she don’t have iPhone, this is her first Apple device). I created iCloud for her, and everything work except FaceTime and iMessage. I need to select region but there is not Serbia on the list (I can’t add number to iPad, our providers support). I try everything, and there still not Serbia on the list. (Please don’t respond with Apple support links I try it all, if someone have solution write to me 😊)