There is a Facebook group called 'Jewish DNA for Genetic Genealogy and Family Research', who can help with advice on how to proceed in learning about your biological grandfather. They have files on how to get started, and also on how to handle some of the more complex issues that come up in these situations.
It is through your close DNA matches that you will be able to zero in on who your grandfather might be, or at least zero in on the family in which he was a member. It is your close DNA matches who are 100% Ashkenazi Jewish who are relevant here. And since you tested on AncestryDNA, those close matches who have family trees are the most useful ones to you.
Lak/Lok/Liak/Lock and Kalon/Kolon in Zagare/Joniskis/Gruzdziai, Lithuania
Lak/Lok/Liak/Lock in Plunge/Telsiai in Lithuania
Rabinowitz in Papile, Lithuania and Riga, Latvia
Trisinsky/Trushinsky/Sturisky and Leybman in Dotnuva, Lithuania
Olitsky in Alytus, Suwalki, Poland/Lithuania
Gutman/Goodman in Czestochowa, Poland
Lavine/Lev/Lew in Trenton, New Jersey and Lida/Vilna gub., Belarus