Things of note:
The 2nd link in the list above is a relative link; as long as you're on a Lemmy-instance, it should link to the right community relative from your instance.
If you get an error saying '404: couldnt_find_community', you will have to search for the community first. For example: /search/q/[email protected]/type/Communities/sort/TopAll/listing_type/All/community_id/0/creator_id/0/page/1. You will get no results, but visiting the linked community should now work. It can take a little bit of time, and posts might not instantly show up. If it still doesn't work, the linked instance might be blocked on your current Lemmy instance. I'd recommend asking your admins for support at that point.