We use the phrase 那倒是 to express agreement with what was said before.
We can state it either at the beginning of a sentence, at the beginning of the second clause of a sentence, or as its own sentence.
The phrase 那倒是 is used only in colloquial speech.
Well, that's really interesting
nà dàoshi tǐng yǒuqù de
如果能找到失败的原因，那就非常好 —— 那倒是！
It would be very good if we could find the reason for the failure —— That’s true!
rúguǒ néng zhǎodào shībài de yuányīn, nà jiù fēicháng hǎo — nà dàoshi