Of Human Dignity and its Ground II

In the last post, I showed an analysis about the ground of dignity in Kantian ethics, with a long argument, of course. However, I realized an issue during the thread, with the help of a Discord group which talks about him: Metaphysics.

I mean, yeah, you could say rational beings are subject of right... But how do you know who's subject of right or not? Basically, that'd be attempting to grasp the thing-itself, something that isn't plausible. Now, I get why some people say that Kantian ethics are individualistic, perhaps in that sense.

Basically, you can't prove even other people like you are rational beings, you just... Use practical reason for that? I mean, you presupose so? Honestly, I liked most of Kantian ethics, along with his perspective of the duties, natural perfection, etc. Nevertheless, this point I consider it an Aquiles' heel to his philosophy. How would Kantians address that gap? Just merely presuposing rational beings, and that's all?

As someone very interested in his Deontology, I believe if it wants to get a more solid grounding, it should have other metaphysical framework. Right now, I get what Starfleet_Stowaway probably wanted me to understand.