Both sentences are correct. "I had not understood the problem until he explained it." past perfect, simple past. "I did not understand the problem until he had explained it ." simple present, past perfect The meaning of these sentences is almost identical, so I wouldn't worry too much about it. For more information about when to use which verb tense, see: https://www.englishpage.com/verbpage/verbtenseintro.html

Hi C-duo, Both sentences are fine, but you could also consider 'I did not understand the problem until he explained it' I hope this helps, Tom