I have w123, w124, and w126 and I never can remember which model has vertical or horizontal for unlock because they are different.

When the button is in unlock position, try to push the button with the key inside the lock. I have seen that problem happens to some MBs (including one of my cars ). Another problem I saw was that the trunk was actually unlocked but needed a strong lift from the owner (lift the lip as Ron indicated). That is due to rust or sticky latch.

Good luck

David