In Opera for Android, by clicking on the Opera icon in the bottom right and logging in.
If you use the tablet mode, you need to use the hamburger menu.
Yes, I was logged in. As I said before I was constantly signed in Opera.
However, other bookmarks from other devices were not appeared. Finally I had to uninstall and reinstalled the Opera app for third time in order to see the bookmarks from other devices appeared. Troublesome indeed but at least I got it done... thanks anyway!