According to Encylopaedia Brittanica all of the tribes of Israel except Benjamin, Judah and Levi have been "lost to history", assimilated into other peoples and no longer have a Jewish identity:
...Following the conquest of the northern kingdom by the Assyrians in 721 BC, the 10 tribes were gradually assimilated by other peoples and thus disappeared from history... - Ten Lost Tribes of Israel
However, when Jesus came preaching the kingdom of God he appears to claim that his only mission was to "seek and to save" these missing "sheep":
Luk 19:9 And Jesus said unto him, This day is salvation come to this house, forsomuch as he also is a son of Abraham. Luk 19:10 For the Son of man is come to seek and to save that which was lost.
Luk_15:4 What man of you, having an hundred sheep, if he lose one of them, doth not leave the ninety and nine in the wilderness, and go after that which is lost, until he find it?
Mat 15:21 Then Jesus went thence, and departed into the coasts of Tyre and Sidon. Mat 15:22 And, behold, a woman of Canaan came out of the same coasts, and cried unto him, saying, Have mercy on me, O Lord, thou Son of David; my daughter is grievously vexed with a devil. Mat 15:23 But he answered her not a word. And his disciples came and besought him, saying, Send her away; for she crieth after us. Mat 15:24 But he answered and said, I am not sent but unto the lost sheep of the house of Israel.
Was Jesus sent to locate and gather in the lost tribes?
I note that some consider the existence of the lost tribes a myth while others, mostly in more modern times consider their recovery part of the messianic hope:
...The mysterious disappearance of the Ten Tribes of Israel nurtured the belief according to which their location will eventually be discovered and they will return to the Land of Israel, as the ancestors of the modern Jews, the tribes of Judah and Benjamin did when the Babylonian empire was destroyed by the Persians. This belief had its roots in the interpretations of several biblical texts, especially I Chronicles (5:26) and various prophecies (Isaiah 11:11-12, among others) as well some references found in the Apocrypha (II Esdras 13:39-50).
The fate of the Ten Tribes was discussed by the sages of the Mishnah and the Talmud. Their opinions diverged between that expressed by Rabbi Akiba, who believed that the Ten Tribes would not return, and that of Rabbi Eliezer, who argued that the Ten Tribes would eventually return (Mishnah, Sanhedrin 10:3; for additional references see Babylonian Talmud, Shabbat 147b, and Numbers Rabba 9:7). The supposed location of the Ten Lost Tribes became a subject of much speculation in itself. Gradually a legend was formed that claimed that the Ten Lost Tribes live in a region situated beyond the miraculous and impassable river of Sambatyon who flows for the six days of the week and stops on Shabbat, when the Ten Tribes are forbidden to travel. References to this theme may be found in Jewish classical texts (Genesis Rabba 73:6; Sanhedrin 10:6/29b). The legend is also mentioned by Josephus Flavius (Wars: 7:96-97) and the Greek author Pliny the Elder (Historia Naturalis 31:24). - The Myth of the Ten Lost Tribes
So did Jesus come to seek and save them? Was that plan thrwarted by the Jewish leadership not recognizing Jesus as the messiah?
I note the following NT passages in which the 12 tribes appear to exist in the "present" (the then present) and in the "future" (the then future, and possibly future to us):
Mat_19:28 And Jesus said unto them, Verily I say unto you, That ye which have followed me, in the regeneration when the Son of man shall sit in the throne of his glory, ye also shall sit upon twelve thrones, judging the twelve tribes of Israel.
Luk_22:30 That ye may eat and drink at my table in my kingdom, and sit on thrones judging the twelve tribes of Israel.
Act_26:7 Unto which promise our twelve tribes, instantly serving God day and night, hope to come. For which hope's sake, king Agrippa, I am accused of the Jews.
Jas_1:1 James, a servant of God and of the Lord Jesus Christ, to the twelve tribes which are scattered abroad, greeting.
Rev_21:12 And had a wall great and high, and had twelve gates, and at the gates twelve angels, and names written thereon, which are the names of the twelve tribes of the children of Israel.