It’s not circular. The authority that enforces a right need not be legal (although I listed many examples related to legal authority for the sake of illustration). It can be cultural or supernatural or natural, to give a few examples.
The point is that the word “right” has meaning. It conveys an entitlement. It’s something more than just an aspiration about how things should be or a judgment about what is or is not moral. And the term “natural right” implies that a particular right somehow derives its authority from nature (which the Enlightenment thinkers who advocated this concept attributed ultimately to a supernatural source).
My question is: in what sense does nature enforce any natural rights? If anything, nature seems hellbent on violating them. We all die, for example. As it pertains to life, the only inviolable law of nature is that it will inevitably be taken away from you. So how is the right to life in any sense protected by nature? From the time we’re born, nature is out to kill us.
But maybe I’m assuming too much. Maybe those who claim natural rights exist merely mean something like: there are certain moral principles that seem obviously correct. Okay. If that’s your view, fine. But you’re talking about something else entirely and the word “right” is just misleading propaganda. That something is morally correct is insufficient to give anyone an entitlement to the morally correct outcome. Which makes labeling these concepts as “rights” misleading or at the very least confusing.