About Yaajooj and Maajooj ( Arabic: يَأْجُوج وَمَأْجُوج‎ )

I want to know that it's said that Yaajooj and Maajooj live behind a really tall and strong wall (made by hazrat zulqurnain ). but if it's so then it means that they are cut off from rest of the world and can't be contacted. So no one is able to tell them about hidayat and Islam.. and they won't be able to accept Islam. Still it's their mistake?
They don't know anything about Islam.. or is there any way for Allah to preach hidayat.
Please clear my mind.