The Ten Lost Tribes aka The House of Israel will be united with the House of Judah to finish up God's final plan of the restoration of the Kingdom of Israel.
This 10 lost tribes thing has been peddled by among others, Herbert W. Armstrong of the Worldwide Church of God as British-Israelism ( the Brits are mostly descendants of the 10 lost tribes ).
Adter Armstrong's death, control passed to a new Pastor General, Joseph W. Tkatch. His son, Joseph W Tkatch Jr, in turn, succeeded his father in 1995.
The Tkatchs instituted a program to abandon many of the non-orthodox beliefs and practices of this church.
They brought the denomination into alignment with other conservative Christian churches.
Church membership dropped from its peak of about 150,000 members in 1986. In the US, its membership slid from 1986 to 1996 from about 89,000 to 49,000, largely because of this shift.
I don't think they teach British-Israelism any longer.