According to Kant the thing-in-itself or noumena is strictly hidden from us and phenomena are conditioned by the categories of the mind such as time, space, causality amongst others. These categories allow for the possibility of experience.
Originally they were described by Aristotle but were seen as natural and not as Kant saw them as pure concepts of the understanding.
It appears that existence as it is for Aristotle a Kantian category.
We cannot say that the noumena exists nor does not exist because that is an application of the category of existence, to the noumena which we insist we cannot have direct access to.
Similarly We cannot say that the noumena causes nor does not cause phenomena because causality is also a category of understanding.
If we cannot say that the noumena at least exists, then should we retain it? That is does the word 'noumena' actually refer to something? Or must we say that noumena is indefinite in all its qualities including existence? Is this even a sensible thing to say? What does Kant say? Or his followers?
If we drop the noumena are we committed to mind dependent reality? This is different, I think from solipsism, for reality is dependent on mind; but also mind is dependent on reality. That is the two terms have a mutual relation of dependence. Is this how Kants sees it? Or his followers?
Presumably this is how Kant is seen as a founder of German Idealism.
I do realise that its not good to mix & match philosophical traditions - one gets something close to bad fusion music but I think it is at least interesting to notice parallels:
Given that Kant was answering Hume skeptism which was profoundly influenced by Sextus Empirucus texts on Pyrrhonism; and that there is evidence that Pyrrho visited India and debated with gymnosophists which, are most likely to have been Jain philosophers perhaps some of their terminology may be useful here:
Anekāntavāda - literally means not-one-attribute-school-of-thought, from which derives their form of relativism & sceptism. Also, syādvāda - means from-a-perspective-school-of-thought
Under the ontology of syādvāda there are seven propositions that examines the complex and multifaceted nature of reality from a relative point of view of time, space, substance and mode.
Two of them sound useful in this context:
syād-asti-nāsti-avaktavyaḥ — in some ways, it is, it is not, and it is indescribable
syād-avaktavyaḥ — in some ways, it is indescribable.
After reflecting on this, I can only suppose Kant uses existence in several ways. Existence as a pure form of intuition allows him to assert the existence of tables and chair etc. We have direct epistemic & perceptual access to this. Existence as applied to noumena is of a different kind. We know that it is not perceptual or conceptual. All we can say about what existence means here (and this is an assertion) is that it the noumena didn't exist then neither would phenomena. If we think of existence as a predicate in logic (which I know is controversial), it needs to be typed logic, so we can distinguish between the two senses. We could, alternatively call it transcendental existence - I don't know whether this matches with Kants terminology of transcendentally.