Is one who performed Umrah in Ramadan and returned to his country, then performed Hajj in the same year considered a Mutamatti' (one performing Hajj At-Tamattu') and is a sacrifice due upon him?

You are not a mutamatti‘ (one performing Hajj al-Tamattu‘), and no sacrificial animal (hadi) is required of you. This is because you entered into ihram (definition) for ‘Umrah (definition) in Ramadan (definition), which is not within the months of Hajj. Even if you had performed ‘Umrah in the months of Hajj, your return to your country after the ‘Umrah would nullify the tamattu‘, because one of the conditions for tamattu‘ is that the pilgrim does not return to their country after performing the ‘Umrah.