God's law is a reflection of His holy, righteous, and good character and it is His instructions for how to act in line with his character, so saying that Yeshua came to do away with God's law is a bit like saying that he came to do away with His character. The way to act in line with God's character existed from the beginning and exists independently of any covenants, and did not change between any of God's covenants. Even if God had made no covenants with man, the right way to live would still be according to His character.
In Matthew 5, Jesus was about to contradict what the teachers of the law of his day were saying about the law, which would have sounded to his audience like he was abolishing the law, so he preceded that by assuring them that he came not to abolish the law, but to fulfill it, which he then proceeded to do six times by teaching how to correctly understand and obey it. Christ came not to end the law, but to show us how to obey it, and gave us a perfect example to follow of how to obey it, and we are told to follow his example (1 Peter 2:21-22). The New Covenant involves God writing His law on our hearts (Jeremiah 31:33) and we are also told to have a holy, righteous, and good conduct (1 Peter 1:14-16, 1 John 3:10, Ephesians 2:10), which means we are told to follow God's instructions in His law for how to act in line with His character.
There is a difference between saying that Gentiles are grafted into Israel through faith in Messiah (Ephesians 2:19) and saying that Gentiles are actually one of the lost tribes.