Greetings. Why is there no source telling us how Jewish communities that were far away from Eretz Yisroel kept yomtov in the days of the first Temple or even the second Temple who could not be contacted by a shaliach or a fire? If they automatically had to keep a second day, why is there no Mishnah that discusses this?
You are asking an interesting question. In the sefer YomTov Sheini Khilchoso in the first chapter of his Mevo, he discusses the history of Yom Tov Sheini, and he says that originally the message was sent either by fire or messenger, but during the first generation after they couldn’t send use the fire method because of the Seduces, if there was a community that had a sage that could properly calculate when Yom Tov should be, that community didn’t keep two days of Yom Tov. It wasn’t until 100 years later, in the generation of R’ Elazar Ben Pidos that Yom Tov Sheini became the established minhag outside of Eretz Yisroel.
I attached a copy of the relevant pages, you can see it here. יום טוב שני כהלכתה מבוא פרק א- ר’ פריד