Hey M_87,
Thank you for your answer, but I still have some issues on this matter. In the second link that you provided there is this note:
* Don't see the Text Message Forwarding setting on your iPhone? Go to Settings > Messages, turn off iMessage, turn it back on, tap Send & Receive, tap Use Your Apple ID for iMessage, then sign in with the same Apple ID used on your other devices.
Because I don't see Text Message Forwarding setting on my iPhone I follow upper mentioned steps. And this is the screenshot's from my iPhone:




I am sorry if the snapshots are out of order but I think you will understand what is happening. I'm trying to sing in to iMessages with the same Apple ID as I have on other devices but problem is I cannot find Serbia in the list of regions here, and my Apple ID on all of the other devices is in region Serbia. I think this is the root of my issues and I don't understand why Serbia is not in this list because it is in every other list of regions for Apple ID