Seems to be solved already from one of your comments, but there's some useful tips in such cases.
For example, if you write in a comment, post, DM, etc. the community handle, you get a community mention you can open locally (at least on browser; I don't use the apps), e.g. [!privacy@lemmy.ml](/c/privacy@lemmy.ml) becoming !privacy@lemmy.ml
Similarly, if you go to https://lemmy.world/search and search for a post's link, it should pull the given post even if it hadn't previously been fetched, thus allowing to view the community too (which also gets pulled if it hadn't before either). For example, searching for https://szmer.info/post/17039481, a post from the Privacy community you wanted to join in, pulls the local copy for lemmy.world, https://lemmy.world/post/54245878.
Another thing, an external community only gets populated when older posts are pulled manually, e.g. through the search function above, and/or when someone subscribes to that community, thus pulling newer ones as they come.