If you take a look at the link below :
https://www.ebay.co.uk/itm/ROVER-75-...gAAOSwXetceRut
You will see there is a lever with a white clasp/clip attached to it. This is the bit you need to get hold of and push downwards. There should be a rod connecting to the outer door handle to this plastic gizm, but I suspect the clip has let it slip off and allowing the door handle to do exactly nothing at all.
So to sum-up, the piece where the white clip/gizmo is the part that will need to be pushed down, this releases the door catch from the body and the door should open.
Hope this helps.