Without recognizing, and understanding about prophecy involving the fate of the ten lost tribes of Israel, it will be impossible to grasp what Paul meant about 'blinded' Israel's restoration in the Romans 11 chapter, and the continued existence of God's promises to His elect remnant of Israel that were not blinded.
One cannot just simply omit the following section of verses Paul gave in that same chapter towards its end about the blinded part of Israel, quoting from the OT prophets...
Rom 11:25-32
25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.
26 And so all Israel shall be saved: as it is written, "There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:
27 For this is My covenant unto them, when I shall take away their sins."
28 As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers' sakes.
29 For the gifts and calling of God are without repentance.
30 For as ye in times past have not believed God, yet have now obtained mercy through their unbelief:
31 Even so have these also now not believed, that through your mercy they also may obtain mercy.
32 For God hath concluded them all in unbelief, that he might have mercy upon all.
(KJV)
It's simple. When "the fulness of the Gentiles be come in", THEN the blindness God put upon the unbelieving part of Israel, will be removed. Before anyone can 'understand', they must first be given to 'hear' (spiritually). Our Lord Jesus showed us this in Matt.13 about those to whom He said His Mysteries of the Kingdom were NOT... given; that their eyes and ears were dull and their hearts waxed gross. But to His Apostles, He told them they were blessed, because for them, they were given. This was the very matter Paul was talking about in that same Rom.11 chapter with the following verses...
Rom 11:7-10
7 What then? Israel hath not obtained that which he seeketh for; but the election hath obtained it, and the rest were blinded
8 (According as it is written, God hath given them the spirit of slumber, eyes that they should not see, and ears that they should not hear;) unto this day.
9 And David saith, Let their table be made a snare, and a trap, and a stumblingblock, and a recompence unto them:
10 Let their eyes be darkened, that they may not see, and bow down their back alway.
(KJV)
One of the definitions for that Greek word for "slumber" is a 'stupor', a lethargic stupor.
The ten lost tribes were scattered to the West where The Gospel of Jesus Christ was first preached outside the holy land. They were already there among the Gentiles that would also 'hear' The Gospel and believe. They both together became one people under Jesus Christ, His Church. So that's about the remnant of the seed of Israel according to election which God preserved. And along with The Gospel, God's promises to Israel remained with that elect remnant of scattered ten tribed Israel, and are now fulfilled among them, with believing Gentiles, as Christ's Church. This... is how the "covenants of promise" have continued with the seed of Israel, the remnant which God reserved. And it is also how the believing Gentiles would be 'graffed' into those "covenants of promise" through Christ Jesus, and both groups would become "the commonwealth of Israel" (Eph.2).
Yet the 'unbelieving' part of Israel, the part Paul showed that God has blinded until the fulness of the Gentiles be come in, they are still... God's chosen people today. They are still His election, which Paul also showed in the Rom.11:28-29 verses.
So what may we expect will happen to that blinded unbelieving part of the seed of Israel once "the fulness of the Gentiles" is complete, and God removes their blindness? We may expect MANY of them to turn to Christ Jesus and believe, because in that time the removal of that blindness means their being given eyes to see, and ears to hear (spiritually). And then, whichever ones among them that still refuse to believe on Christ Jesus once their blindness is removed, then those we may expect to be 'cut off'.