I have studied both Hebrew and Greek at the graduate level. I rarely say anything I can't back up.
ἔννομος, ον (s. νόμος; Pind., Aeschyl. et al.; ins, pap, Sir prol. ln. 14; Philo, Abr. 242, Poster. Cai. 176; Jos., Ant. 19, 302; SibOr 3, 246; Just., D. 47, 4 ἔ. πολιτείαν; Ath., R. 70, 23 al.) pert. to being in accordance with law, legal, lawful. ἔ. ἐκκλησία Ac 19:39 could, acc. to the context, mean a legally convened assembly in contrast to a mob, but certain features of the word’s usage (Lucian, Deor. Conc. 14; SIG 672, 37 [II B.C.] ἐν τᾷ ἐννόμῳ ἐκκλησίᾳ) suggest the interpr. regular assembly in contrast to one called for a special occasion (s. IBM III/2, p. 141; WRamsay, Pauline and Other Studies3 n.d. [1906] 203–15).—Subject to law, obedient to law (Aelian, VH 2, 22 v.l.): ἔ. Χριστοῦ subject to the jurisdiction of Christ 1 Cor 9:21 as opposed to Mosaic jurisdiction (B-D-F §182, 3; Rob. 504—Proclus on Pla., Crat. p. 93, 5 P., the contrast ἔ. and ἔκνομος). The entire vs. can be rendered: ‘I identified as one outside Mosaic jurisdiction with those outside it; not, of course, being outside God’s jurisdiction, but inside Christ’s’. Fr. a purely linguistic point of view one can also transl. ἔννομος true to the law, upright, in the right (so ἔ. in Aeschyl., Suppl. 404; Pla., Rep. 4, 424e) acc. to the judgment of Christ.—CDodd, Studia Paulina (JdeZwaan Festschr.) ’53, 96–110.—M-M. EDNT. TW.
Arndt, W., Danker, F. W., Bauer, W., & Gingrich, F. W. (2000). A Greek-English lexicon of the New Testament and other early Christian literature (3rd ed., pp. 337–338). Chicago: University of Chicago Press.