Reference: Fataawa Ramadhaan – Volume 1, Page 198, Fatwa No.139
Question: If I wanted to fast but was unable to wake up before the adhaan of the second Fajr, is it permissible for me to eat after the adhaan knowing that this is a voluntary fast?
Response: If the situation is as you have stated, then do not eat or drink after the adhaan [of the second Fajr] so long as you wish to fast, even if your fast is a voluntary one. If you were to eat after this Adhhaan, your fast would become nullified.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.