Originally Posted by invisible

Seems OP did not do any research and wants to get answers day before departure.

In short - he can't get in to any lounges airside or landside because he needs to have boarding pass for the departure (not arrival) flight which he won't have.

UA does not issue boarding passes from SIN departure online or via mobile app. You have to go to the counter to get your BP. Counter opens only 3h before departure. You can't get BP at transit desk either.

So OP has to go landside and come back 3h before departure. What to do that time - there are number of threads in this forum.

Good luck.