Just gonna share what worked for me...
1) promotion threads... but don't over spam or it may turn off some potential readers. And when promoting, post a sample panel or cover page that may be interesting...
2) community engagement... read, like, sub, comment on other people's work, participate in threads (but steer clear as much as possible from religion and politics. You might lose at least 50% of potential subscribers)
3) use social media... Facebook, Instagram, YouTube, deviantart, etc.
4) join niche sites that may have certain groups of people that may be potentially interested in your stuff.