Short answer : a thing in itself ( a monadic substance) does not depend on its ( spatio-temporal) relational properties to exist ( to the contrary, relations depends on substances; relations are accidents ) ; but the things we experience need spatio-temporal properties to exist; so the things we experience are not things in themselves.
When Kant thinks of " things in themselves" what he has in mind is leibnizian monadic substances.
According to Leibniz, space and time are ordering relations applying to monads.
if it is the case, these spatio-temporal relations depend on the existence of these monads.
Kant's argument could be reconstructed like this :
(1) If space and time were relational properties of things in themselves, then space and time could not be conceivable in case the things that are in space and in time were suppressed. For relations having an absolute extra-mental being are not conceivable without their " in themselves" relata.
(2) But, in fact, we can conceive or represent ourselves space and time, spatio-temporal relations, without anything being related by them. I can represent myself an empty space and an empty time.
(3) So, the things that are related by spatio-temporal relations are not things in themselves ; they depend on spatio-temporal relations to " exist" ( and not the other way round) ; they are not monadic substances, but " phaenomena" ( mind dependent entities).