We bought a breath monitor which is a desk/plate (about 20x50cm) with a small device attached via a long cable. You put the desk under the mattress and hang the device somewhere on the crib. When it's on and the kid would stop breathing, the device starts beeping really loud.
It already triggered a few times for us, luckily in all cases it was because we carried the baby from the crib a forgot the turn off the breath detector :-D
I have the Babysense 2 Pro, which may not be exactly the same brand as OP is describing, but does fit their description.
Obviously, it's one of those things where most people will never actually know if it works or not (thankfully), but based on the fact that it goes off without fail every time I forget to turn the damn thing off after picking the baby up fills me with confidence that it does do what it's supposed to.
It already triggered a few times for us, luckily in all cases it was because we carried the baby from the crib a forgot the turn off the breath detector :-D