For everyone in here wondering about the missing communities from other instances, i hope to give you some insight.
Each lemmy instance, like this one, acts as an server for multiple communities, posts and users. You can create a user, which lives on a specific instance and browse different communities through an instance.
Normally, you only see communities from your local instance. If you're interested in other communities, you can look them up on the lemmy community browser (thanks to feddit.de) and later type in the link to a post or community into the search on the top right.
it always took me two tries to add a community, maybe a later update does this better.
After you found your community through this instance, you can subscribe to it with your user and create your personal frontpage.
You may not see all the posts and comments after you added the community. This seems to be normal behavior.