when is the correct time to say ameen when reciting the fatiha behind the imam, should we wait until we finish the surah behind him which will delay us saying ameen as we didnt finished yet or should we say it with the congregation between the last two ayat and then finish the surah?

Ustadh AbdulAziz al-Haqqan

Say amin with the imam when he finishes al-Fatihah, even if you are in the middle of it, then complete your recitation. Ibn Qudamah rahimahullah said this does not cut your recitation.