You should call the FTL hotline at the opening of check-in, i. e. 23 hours before departure. I'm not sure whether you'll be able to check-in for the LSY-FRA leg but for FRA-SIN it should work fine. This way, I have never had problems getting an exit row seat for flights from Germany. Enjoy your trip!
I called and was able to get seats with extra leg-room and no one in front of you (the ones behind the lavatory). However, no way to get the "real" em. exit seats.
Thks for the tip ^