Unable to set region for Messages on iPad

Hello,

I have problem when setting up iMessage, region Serbia could not be found for iMessages, but it is registered for Serbia on startup. Sim is in iPad and it was working in old iPhone before. Sim Internet is working, but iMessage are not. When I select Croatia , setup is ok, but I can only receive iMessage not regular ones...

What can be solution?

[Re-Titled by Moderator]