How to access older posts in a forum channel?

async function countNewGallery(client) {
    const galleryForum = (await (await client.guilds.fetch(guildID)).channels.fetch(galleryChannelID));
    const options = { "limit":100, "cache":true};
    const threads = (await galleryForum.threads.fetch(options)).threads;
    console.log(threads.size);
}