The promises of the New Covenant in Jer 31 are Gospel promises and Paul by inspiration says Gentiles were included recipients of those promises, but it was a Mystery Eph 3 :4-6
4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;
6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
No, that's YOU trying to make these complementary to each other. The "mystery" was never the New Covenant. It's not a mystery for Jeremiah does not hide anything in his prophecy. It is clear who is included in this covenant. It is clear this covenant is between God and the twelve tribes of Israel. It is clear that Jeremiah describes the 'gist' of this covenant, which is God forgiving the sins of His covenant people Israel. It also makes reference to the Mosaic Covenant and that covenant between God and the Hebrew people He delivered out of bondage in Egypt.
If the New Covenant does not include non-Hebrew Gentiles and there are those who re-interpret Jeremiah's prophecy to include non-Hebrew Gentiles, then this inclusion after the fact that the covenant does not include non-Hebrew Gentiles, then any teaching that seeks to insert a people that the prophecy does not mention has caused Jesus to "destroy the Law" (Matt. 5:17), and this in and of itself violates the Old Testament prophecy by Jeremiah thus making Jesus Christ a liar.
It is clear who the New Covenant describes as the participants of this covenant and the parties include the God of Abraham and the House of Israel and the House of Judah, and neither House includes non-Hebrew Gentiles. Both Houses when taken together are the twelve tribes of Israel. Here is the preamble:
31 Behold, the days come, saith
the LORD,
That I will make a new covenant
With the house of Israel, and
with the house of Judah:
Jeremiah 31:31.
The New Covenant includes the LORD, and the House of Israel and the House of Judah, and neither are non-Hebrew Gentiles. You and others who add to God's Word violate the Word of God as written, and to claim the inclusion of non-Hebrew Gentiles when this covenant clearly states Israel and Judah as the parties involved in the New Covenant God has promised through His prophet, Jeremiah, and such teaching that violates the clearly written text of Scripture is to be rejected as false.