I bought a powder to put over my cat’s dry food

Not heard of that before - I hope it works.
The biscuits I was thinking of are these:
Not sure if you can get them where you are. They’re not cheap, but my cats absolutely love them and they do a great job of cleaning all the crap off their teeth 😊
It might be worth a trip to the vet too because they can get a hard tartar build up, which doesn’t smell too good.

It's either the powder or 'dental floss for cats'. Ok kitty open your mouth 🙀

Hope it works 😃