There is a Facebook group called "Jewish DNA for Genetic Genealogy and Family Research", where you should post your questions about identifying your maternal grandfather. There are persons there in that group who have experience solving these sorts of problems, using their understanding of Jewish DNA results and knowledge of Jewish genealogy through records. They may also be able to tell you who you may hire to solve this mystery.
It is the top 20-30 Jewish DNA matches for both your and for your sisters who will be the matches that aid you in this search. You don't have to go through all 16000-20000 Jewish matches to solve your problem. It is important to be able to understand DNA centimorgans lengths and number of shared DNA segments to zero in on who is a descendent of this grandfather or his siblings. Ancestry results are also useful because some matches have their family trees up for viewing.
The group also has advice on how to handle some of the more delicate situations that arise from identifying these unknown relationships. Not everyone who does DNA testing is comfortable finding out their long deceased relative had unknown children.
Good luck with your search.
Lak/Lok/Liak/Lock and Kalon/Kolon in Zagare/Joniskis/Gruzdziai, Lithuania
Lak/Lok/Liak/Lock in Plunge/Telsiai in Lithuania
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