How come when I try to comment or upvote on a post from another instance it says I am not logged in.

As an example, OP was cross-posted from https://vlemmy.net/post/388759. This post lives in the community https://vlemmy.net/c/chat.

An equivalent to the URL https://vlemmy.net/c/chat is https://vlemmy.net/c/chat@vlemmy.net (‘/c/chat’ as seen from ‘vlemmy.net’). If you have no account on vlemmy.net, these pages will display you logged out.

If your account lives on feddit.de, you can visit the same community while being logged in, using https://feddit.de/c/chat@vlemmy.net

Similar to reddit’s /r/syntax, you can do this: /c/chat@vlemmy.net. This will visit the community @ that instance, but while being logged in to your https:// home instance.

TLDR: Whenever you appear logged out, make sure your URL starts like this: https:// home instance