How to refit main door hinges on Beko DC212W cooker?

I'm repairing my partner's Beko DC212W cooker. I have removed the main (lower) door to replace its inner glass, its striker peg and the main rubber seal.

So far so good - replacements made OK.

I also got two new hinge units and retained the machine studs from the old hinges (I unscrewed them from the oven chassis).
When I tried to fit the new hinges to the oven body, I found that - apparently - there was nothing left in the holes in the chassis for the studs to screw into!

What the heck happened (and, how do I proceed)?

If there were captive nuts or similar fixings behind the panelling, I was unaware of such and, in any case, I can't see how the rear of the front panelling could be accessed without dismantling the entire cooker...help!