Sozomenus described the religion of the Saracens in the 5th century saying that they descended from Ishmael and their religion if any different from Judaism was only because of the passage of time and their interaction with non-Jews.
From this we understand that the Ishmaelites should have belonged the same Hyrcanian religion as Judah ben Tabbai albeit of a different minhag due to the years of separation and interaction with non-Abrahamites. This causes us to wonder why Sebeos said that the Jews from Edessa could not convince the Ishmaelites of Tachkastan to unite with them in religion!?
The problem perhaps was that the Ishmaelites had existed as Hagarim without teachers for so long they were reluctant to accept Jews as their teachers.