You might have to enlist the help of a locksmith, but hopefully not as their call-out fees are high. Is there any way a child's hand might reach inside and fish them out for you?

Or you may also use a magnet if the mailbox is not deep.

Call Mister Minit they can produce the dupliate key. Or ask the Concierge of the building he must have a solution.

I’d ask the concièrge and if he can’t help then contact the Régie.  Better to get an officially issued key so that if ever you move out you will have an “authentic” key to hand on to the next tenant.

Or you can try one of those bendable shirt hangers. Bend it like a hook and see if you can fish out the keys. Thats what I did anyway.