Without taking the tiles down do they make some kind of a glue gun to squirt up in between them?
What is the reason why the tiles are sagging? Have they taken on moisture, or have they been improperly installed? The root of the problem needs to be identified and corrected first, otherwise the problem might be ongoing.
Is this a drop ceiling or not? Is there a grid the tiles sit on? A photo would help.
The tile is old and when I try to screw it to secure it it wants to fall apart so I’m looking at gluing it. How can I glue it without taking the tiles down do they make some type of tool to squirt it up in?
If the tiles are disintegrating, this is a sure sign that they need to be replaced with new. Attempting to re-secure them in place with glue is not going to solve the problem that is being caused simply by age. The original adherent that was used in the manufacturing of the tiles has dried out. Liken it with trying to glue together clumps of brown sugar.