It's vitally important to be active in writing communities if you want people to read your work. Not just for fanfic writing, but any sort of writing that's self-published.
(The rest of this post will talk about fanfic writing, because that's all I have experience with.)
Like you said in Discord, online you only have the fandom, the title, and maybe a summary if the website has space for it. There's no guarantee of quality or anything else to draw the reader in. But if you go around and talk about your story in applicable threads, or even just go around and review other people's fics and answer discussions, your name gets out there. People start to recognize your name, and they (maybe) see what sort of person you are as a writer. Talk about your story enough, and people will check it out to get the context of what you said.
While on forums, you also have the benefit of using your signature as an advertising space. I know back in the old days of fandom, everyone had some fancy banner to catch everyone's eye. I think those are beginning to fall out of use (haven't seen them here or on SPPf, and I don't go to other forums much). What people do instead is put their fic's title in their signature with a link to the thread, and some one-sentence summary. But again, by interacting with the community and getting your signature out there, other members see your fic's title and summary, and check it out if interested.
It's best to interact with the community that you're trying to get readers from. Otherwise, no one knows who you are, and very few people are willing to take the gamble on reading your fic.
EDIT: Forgot to mention that by being active on other sites while talking about your story, you'll get readers to go to where your story's posted to read it. As a personal example, by seeing one person on Tumblr reblog art of my favorite character from another member's story, I checked out that other member's story when I never would have known it existed. Now I'll read it, review it with my own name attached, and (once I actually have stuff posted!), people will learn about my own stories and become interested.