There are two explicit reports from Ahmad on this question. The correct view in all such matters is that a legal ruling does not become binding unless one has the opportunity to learn it, and that a person is not required to make up what he did not know was obligatory (1).
It is authentically established that some of the Companions ate after dawn had broken in Ramadan until they could distinguish the white cord (2) from the black one (3), yet the Prophet—may Allah bless him and grant him peace—did not order them to make up the fast (4).
Among them were also those who would remain in a state of major ritual impurity for some time without praying, because they did not know that prayer was permissible with dry ablution, such as Abū Dharr, Umar ibn al-Khattāb, and Ammār
(1) N, M: “so long as he is unaware that it is obligatory.”
(2) A, B, M: “the thread.”
(3) A, B, M: “from the black thread.”
(4) The hadith of Adī ibn Hātim and Sahl ibn Sad—may Allah be pleased with them both—is found in al-Bukhārī, 6/26, Book of Tafsīr, chapter on Sūrat al-Baqara: “And eat and drink until the white thread becomes distinct to you”; and Muslim, 2/766–767, Book of Fasting, chapter explaining that fasting begins at daybreak. Muslim’s version of the hadith of Adī reads: He said: When the verse, “Until the white thread of dawn becomes distinct to you from the black thread” [Sūrat al-Baqara, 2:187], was revealed, Adī ibn Hātim said to him, “O Messenger of Allah, I place two cords beneath my pillow, one white and one black, so that I may distinguish night from day.” The Messenger of Allah—peace and blessings be upon him—said, “Your pillow must indeed be very broad! It means only the darkness of night and the brightness of day.” The hadith is also found in Sunan Abī Dāwūd, 2/408, Book of Fasting, chapter on the time of the predawn meal; and Sunan al-Dārimī, 2/5–6, Book of Fasting, chapter on when one eating the predawn meal must refrain from food and drink.