I have 3 different models and I have only ever managed to open 2 of them.
One is like described above, the entire button unit should be rotated anti-clockwise, will then lift off, and then the cistern lid can be lifted
The 2nd type has its two buttons removable. Pulling one out enables you to get at the 2nd button, and when both removed, the cistern lid can be lifted.
The third type does not have lift off buttons and any time I tried to rotate the button unit, I was able to get sufficient grip, but got to the point where I felt if I applied any more pressure, something would break, so I abandoned every time.