Unlocking Inspiron 2 in 1 when in tablet mode

Hi

I have an Inspiron 15 7579 2-in-1 laptop and it is set to require Ctrl + Alt + Delete to unlock.  This works fine when I'm using it in laptop mode as I just enter the key combination and type in my password as usual.

In tablet mode (with the keyboard folded underneath), the lock screen offers the instruction "Press and hold the power button, and then press the volume down bottom to unlock.  (Or you can unlock by pressing Ctrl + Alt + Delete.)"  This does not work - it just puts the laptop into sleep mode as soon as the power button is pressed.  I have tried reversing the pressing order, i.e. press and hold volume first then press power.  I have tried changing the function of the power button to do nothing in control panel power options.  And then the button combination still does not unlock - it does nothing.

I've also tried swiping the touch screen - no response.  Nor is there any option to enable the on screen keyboard.

The power and volume buttons all work correctly for their normal designated functions, and the touch screen responds correctly in normal situations.

All of the drivers are up to date, according to Dell diagnostics. 

It does unlock with a simple screen swipe when Ctrl + Alt + Delete is not enabled, but unfortunately disabling this is not an option for me.

Would anyone have any ideas on what to try next?

Many thanks!