Hi, it's very strange, I found your page with posts and it said 1.5k subscribers, and when I joined I am presented with an empty community (0 subscribers) and no posts. I checked many times that I am in the right place so maybe I did something else wrong?
I comment on older posts on Lemmy, the same way I do on all Fediverse platforms, I copy and paste the post's federated link into the site's search bar.
This is how you upvote, downvote, comment and so on, when the posts are "invisible" to you.