The text reads in part: ἐκ τῶν ἑπτά ἐστιν. We have an occurrence of ἐκ + the genitive in this construction, which can mean "out of" or "forth from." Mounce also gives the senses: "of, out of; from, away from . . . " See https://billmounce.com/greek-dictionary/toc/epsilon
While some commentators construe the genitival construction here as a partitive/wholative or genitive of the whole--it seems reasonable to view ἐκ τῶν ἑπτά ἐστιν as a genitive/ablative of source. As Solomon [Landers] pointed out, other translations besides the NWT think the genitive in Rev 17:11 is a genitive of source and I agree. That is all that needs to be said at this point. The NWT is not biased in its rendering of Rev 17:11.
Henry Alford writes (Greek Testament Critical Exegetical Commentary ): "He is ἐκ τῶν ἑπτά,—not, 'one of the seven,' but, the successor and result of the seven, following and springing out of them."
Barnes' Notes on the Bible: "and with the strictest propriety it could be said that it was 'of the seven,' as having sprung out of the seven, and as perpetuating the sway of this mighty domination."