The statement that "no access to the noumena is possible" might not be a fair interpretation of Kant. It would be true to say that we cannot understand the noumena through the categories of the understanding and we cannot experience it with the forms of sensibility. At the same time, you are right to assert that for Kant freedom only applies to the noumenal.
The relationship between this and ethics is first considered out in the Third Antinomy but hinted at by Refutation of Idealism. In these texts, it is basically that room is left for freedom in the transcecndental (as Kant uses the term). Again, note that Kant has not said that this realm is wholly inaccessible. It is a domain of reason (remember the title is Critique of Pure Reason). But Kant does not spell out reason's access except insofar as reason has freedom.
Chronologically, Kant's next work to consider it is the Groundwork -- especially section III which gives a rather different account than the third antinomy for how ethics is possible. There, Kant needs to prove the existence of a being that is rational and has a free will. Because only such a being is capable of the ethics described in the groundwork. Most contemporary Kantians don't depend on this framework and instead make their own proofs. Korsgaard, for instance, maintains that we engage in acts of rationality and thus commit ourselves to morality which follows from reason. (I will leave this merely as a remark as to how she tries to ground it). The problem of the determined phenomenal realm is in the background here but not a central feature.
Next, we have the Critique of Pure Practical Reason that gives a different argument for why we have moral freedom that addresses again the problem of a determined phenomenal realm. I think this one is better liked than the Groundwork version. But I don't honestly remember how.
The Metaphysics of Morals follows, and it briefly reconsiders this in the introduction to the combined volumes and in the introduction the Doctrine of Virtue. In there, it affirms the distinction again but argues that we are free and noumenal as rational creatures using the existence of reason and its applicability to both domains in different ways as an argument for morality.
In all honesty, the best treatment, however, occurs in the Religion with the Bounds of Reason Alone. While some people thing this is an appease the censors book, I think Kant genuinely believes what he's writing. And the topic he's considering is whether the will's prior decisions can make morality impossible or more difficult. Here, he considers the choice to be moral that has to precede all other choices (under the description of conversion) and the need for an exemplar who has made the moral choice (presumably a reference to Christ). We also see here an account of moral law where the moral law is only "given by God" but truly given by the subject's own reason -- projectable into the idea of God as a type of crutch.
The noumenal's ability to act as cause in the phenomenological is one that is inaccessible because the categories of the understanding cannot understand it and the forms of sensibility cannot experience its actions -- only its effects. Consequently, the reality of its activity must be an article of faith for rational beings (that and the correspondence thesis that Kant repeats in several places which maintains that even though the world is unjust, there is a God who exists who will even out justice in the next life).