Paul’s reference to not boasting against the branches (Romans 11:18) has to do with not boasting against the Jewish people as if God had replaced the Jews with Gentiles. God did not cast off the Jewish people in exchange for the Gentiles. Instead, God has grafted believing Gentiles into the faith of Christ with the Jews who believe the gospel. Jews who do not believe the gospel are cut off from covenant relationship with God.
Paul tells us that the Jews who have been cut off will be grafted back in again if they will believe the gospel. Paul does not say “WHEN” they believe, but “IF” they believe they will be grafted back in for God is able to graft them in again. This is not a reference to a resurgence of a national status as a covenant nation, but a reference to individual salvation.
The gifts and callings of God are without repentance. Even though Salvation has come to the Gentiles through Israel’s fall (as a covenant nation), the call to be the people of God still stands for all Jews who will believe in Jesus Christ.
Therefore the unbelieving Jew is not outside the saving sphere of God’s grace. God did not replace one ethnic group with another, but extends the hope of salvation to all.
The point of the not boasting against the branches is that God has not cast away the Jews in favor of the Gentiles, but through the fall of NATIONAL ISRAEL as a covenant nation, salvation has come to all, and because of the mercy shown to the world, God will be merciful to the Jew as well, granting life and salvation to those who will believe the gospel.