The problem here is that letter boxes are common property. I’ve not yet seen an apartment letter box that’s part of lot property on a strata plan.
And owners/occupiers are not supposed to repair common property – at least not without the OC’s permission. And the OC/Strata Manager won’t even talk to the OP.
Still, I agree it seems a relatively simple thing to just get the lock fixed by a locksmith and worry about the OC/LL’s reaction later. If the OP is willing to do that.
But the question of the thread is still valid though. That’s about being able to contact the OC/Strata Manager. And there will be other situations that are not so simply “solved”.