After some tests… yeah, collisions are not going to work correctly with that sort of code and using OnTriggerStay will throw the same results according to Unity documentation, you’ll need to find out another way, the best idea I have right now is to use a component called Platform Effector 2D, the issue is that you won’t be able to get down when at the top of a ladder, so you’ll need to find a way to solve that.
My final suggestion would be to study how others games do it, Megaman X has a lot of stairs and they work perfectly, same goes for Mario games, you might want to look into them.