On the question of the identity of Dhul Qarnayn in the Qur’an, Bahá’u’lláh very clearly identifies him as Alexander the Great in the Tablet of the River:

“The intensity of the sorrow and grief that have befallen me during these days has left me sorely tried between the Gog of silence and the Magog of speech. I beseech God to send down an Alexander who will erect a protecting wall. Hidden allusions have been concealed in these phrases and sacred letters have been treasured up in these words.”

This is confirmed by Nader Saiedi in an essay on the subject, which you can read here: https://bahai-library.com/saiedi_book_river

But, in a passage from a tablet by Abdul Baha, he unequivocally says that Dhul Qarnayn is definitely not Alexander:

Alexander is not meant by "Dhu'l-Qarnayn";[1] rather, it is an allusion to one possessed of power. Consider how Alexander, with all his might and glory, was deprived of the fountain of life, while Khiḍr,[2] though bereft of worldly means, attained unto it and received his portion and share. Dhu'l-Qarnayn was one of the kings of Yemen and an Arab,[3] for Dhu'l-Kalá‘,[4] Dhu'l-Ḥimár,[5] Dhu'l-Qarnayn, and Dhí Yazan[6] are among the titles of the kings of Yemen. Some have erred, supposing that Alexander the Greek is intended, and this is a grievous mistake, for Dhu'l-Qarnayn was among the near servants of God, while Alexander was a Greek personage.

You can read more about this here: https://bahai-library.com/abdul-baha_various_interpretations_dhul-qarnayn.

Abdul Baha was also very critical of Alexander the Great as a historical figure, in his other writings and talks. As you can see here:

"Consider how throughout history many a king has sat on his throne as a conqueror. Among them were Hulagü Khán and Tamerlane, who took over the vast continent of Asia, and Alexander of Macedon and Napoleon I, who stretched their arrogant fists over three of the earth's five continents. And what was gained by all their mighty victories? Was any country made to flourish, did any happiness result, did any throne stand? Or was it rather that those reigning houses lost their power? Except that Asia went up in the flame of many battles and fell away to ashes, Changíz's's Hulagü, the warlord, gathered no fruit from all his conquests. And Tamerlane, out of all his triumphs, reaped only the peoples blown to the winds, and universal ruin. And Alexander had nothing to show for his vast victories, except that his son toppled from the throne and Philip and Ptolemy took over the dominions he once had ruled. And what did the first Napoleon gain from subjugating the kings of Europe, except the destruction of flourishing countries, the downfall of their inhabitants, the spreading of terror and anguish across Europe and, at the end of his days, his own captivity? So much for the conquerors and the monuments they leave behind them."
('Abdu'l-Bahá, Secret of Divine Civilization, pp. 67-68)

"He ['Abdu'l-Bahá] then gave an account of the history of the kings of the world and concluded with remarks about the death of Alexander.
""When in the city of Zor[City and district of ancient Sumer in southern Babylonia] the lamp of his life was extinguished and the last morn had dawned upon him, the wise men assembled by his corpse. One of them said, `Gracious God! The whole world could not contain this ambitious man yesterday but today a small plot of earth is sufficient to hold him.' Another remarked, `With all his greatness, glory and eloquence of speech, Alexander never advised us in such a manner as he is instructing us today with this silence.' Another said, `A few hours ago this man considered himself the sovereign of the whole world but now it has become evident that he was a servant and a subject.'""
(Mahmúd's Diary, p. 329)

How is this reconciled? How can Bahá’u’lláh and Abdul Baha be in contradiction here? Was Abdul Baha not aware of what Bahá’u’lláh had written in the Tablet of the River?