Generally the fediverse is not designed to import identities. Every instance has its own set of identities and you can’t log in across instances like that. You can access posts, but can’t use another instance to post. It’s a matter of trust. The original instance has no way to trust the second instance with your credentials/the authority to post on your behalf.
You need to follow people for their posts to federate to your server. Posts won’t federate until you’re following a user and they make a post, or boost a post. Also, old posts won’t federate unless they are boosted/reblogged/shared. This is also how Mastodon works.
I recommend following as many people as you can, especially people who boost a lot, and following the people who are being boosted.
It takes a lot of effort to start the process up.
Also make sure that the AP and federation features are turned on in the env file, as i believe by default they are off with pixelfed.