First off make sure that you have turned the key in the door to the open
position, then climb inside, hold the  door handle inside as if you were
about to exit the car, now for the fun bit, while doing this bang your
shoulder into the door a few times and if you are lucky it will open. I have
a Citroen synergie and the doors on that seem to stick themselves closed
sometimes within a few days of not using it.
I like you have oiled and wd40'ed  silicone greased it but it still seems to
be able to stick shut within days.  Gentle pressure from the inside might
just do it .
I had a 505 estate many years ago and the sunroof on that was a slide back
type and that would stick itself stuck on the rubbers and would need you to
climb in the back put your feet on the back of the seats and pull like mad
to open it.
GGJ

If the mechanism's like the one in the 205 there's a spring that fails
so one of the linkages sticks halfway.  If you can get the door panel
off it needs to be pulled downwards.  I fixed ours using an extension
spring and a length of steel wire to one of the holes in the bottom of
the door used to attache the panel.

If it feels like it's unlocking though (especially from the inside) try
a good hard push from the inside while pulling the handle - the grip on
the door seals when they get stuck (especially iced up) has to be seen
to be believed.