Look at
https://heraldofgrace.org/the-new-covenant-people-of-god-and-dispensationalism/
Dispensationalists expect that the multitude of physical Israel will be restored in the Millennium. This millennial Israel will include a rebuilt temple and its sacrificial system. It is this restored physical Israel which is seen to be under Jeremiah’s New Covenant, not the Church.
J. Dwight Pentecost says “… the new covenant of Jeremiah 31:31-34 must and can be fulfilled only by the nation of Israel and not by the Church…the covenant stands as yet unfulfilled and awaits a future, literal fulfillment.”6 Charles C. Ryrie states, “The New Covenant is not only future, but millennial.”7 Likewise John Walvoord tells us “…the premillennial position is that the new covenant is with Israel and the fulfillment in the millennial kingdom after the second coming of Christ."
"The New Covenant therefore is not the possession and privilege of the Church. The Dispensationalist separates the Church and Israel as two peoples of God, each with their own promises and destinies. According to Ryrie, “A dispensationalist keeps Israel and the Church distinct…a man who fails to distinguish Israel and the Church will inevitably not hold to dispensational distinctions.”9 “If the Church is fulfilling Israel’s promises as contained in the New Covenant or anywhere in the Scriptures, then premillennialism is condemned.”
But as happens to dispensationalist when "rightly dividing" New Testament scripture, the dispensationalist doctrine that the New Covenant applies only to physical Israel in the millennial kingdom, and not to the church, runs headlong into New Testament doctrine.
"Not that we are sufficient of ourselves to think any thing as of ourselves; but our sufficiency is of God;
6. Who also hath made us able ministers of the new testament; not of the letter, but of the spirit: for the letter killeth, but the spirit giveth life." II Corinthians 3: 5-6
Paul was the Apostle to the Gentiles - Romans 11: 13, I Timothy 2: 7. All who come to faith in Christ through the writings of Paul have received his ministry of the New Covenant.
In Romans Chapters nine through eleven, Paul uses the word "Israel" in two different ways. One "Israel is what he calls "Israel after the flesh," or the multitude of physical Israel, which is dispensationsm is all about, in I Corinthians 10: 18, and the other way he used "Israel" is in a redemptive sense, as a believing remnant.
If the New Covenant applies only to a future, millennial, restored physical and national Israel, how could Paul be a servant of the New Covenant and at the same time the apostle of the Gentiles?
And again dispensationalists just ignore this disagreement with the New Testament and assert their "authority" in place of that of the New Testament.
In Romans 11: 26 Paul defines "All Israel" in terms of salvation, not by race or nationality.
The complete remnant of believing Jews are, under Christ, made equal to and combined with the full remnant of believing Gentiles who together make up the Good Olive Tree in Romans 11: 24 which is the Israel of God.