Honestly, I find persistence is key. I advertise my work a lot on social media. I have a Facebook page and Instagram dedicated to my author-based works and happenings. For the completed novel on my Wattpad, I actually dropped a little money running a Facebook ad campaign to really promote it. But I wouldn't say I recommend that, to be quite honest.
Forums, Discords, they all help. But the main thing is to remember: you get out what you put in. Yes, it might feel like you're pouring hours into your work and few people are noticing, but keep at it. Make it so it is impossible for your work to be ignored! Make it stand out from the masses!
Alongside this, give back to the community that you're asking something from. Readers (and people in general) seldom give up their time without getting something back. Now, you might think the joy of reading your work ought to be enough. It would be...if your audience were primarily readers. But the fact of the matter is you are pitching your work to an audience/market that is saturated with writers-who-read-too as opposed to readers. "Sub-4-subs" mean nothing. Do something of value for others. For example: I am a copy editor as my day job. So, I offer editing services sometimes in return for someone reading my work and giving me feedback (editors are only human. We aim for 80% perfection, haha. Even we make grammar errors and miss typos sometimes!).
As silly as it sounds, what I'm trying to say is: don't just promote your work. Promote whatever kindness you're willing to give to others in return! Maybe an in-depth read of one chapter with X amount of comments as feedback beyond "I love this", for example. Banners or covers, perhaps, with a payment system of reading a chapter of your work?
I hope this helps! I'll round off with a cheeky plug for my current ongoing work, oh ho ho ho!