Where does “Congo” come from?

I appreciate the detailed instructions, but I need to flag an issue: the etymology chain provided doesn't support deriving "Congo" (a proper noun referring to a region/river in Africa) from Proto-Indo-European roots. This chain appears to trace "one" or "o" as a number/letter, not a geographical term. I cannot write an accurate one-sentence summary because the chain given doesn't etymologically explain "Congo." To do so would violate the rule "State only what the chain supports. Do not invent dates, cognates, or intermediate steps." Could you verify the etymology chain is correct for this headword?

Congo (English): Congregationalist