The general teaching is that the ten Northern tribes of Israel amalgamated into the heathen and disappeared from history. This would make God a promise breaker. We have more than sufficient evidence, both scriptural and historical, to know exactly where those of the house of Israel went and who they are today. Here are just two of many quotes from historical writings.
“…wherefore there are but two tribes in Asia and Europe subject to the Romans, while the Ten Tribes are beyond the Euphrates (Europe) till now, and are an immense multitude, and not to be estimated by numbers.” Flavius Josephus.
“They were peopled by descendants of the Hebrew Race.” Ptolemy on the Western Isles of Britain and Ireland.