Try lots of different platforms.
Some series get great results from instagram, others from twitter, others from tiktok and others from Reddit etc. You might not know which platform you'll do best on until you actually try it.
Test different marketing approaches.
If you pay attention to the Tapas front page, you might notice that you'll see the same comic switch through multiple banners over the course of a week. This is a common technique companies use with adverts. If a banner isn't getting clicks on a particular platform, they'll change the banner and see if that gets more. So if a banner that makes the series look serious and action-packed isn't getting interest, they'll try one that makes it look like a character-focused drama, or a bold, energetic comedy. Keep trying things.
Learn from Youtube Thumbnails and clickbait:
Simple, bold, has one or more people with an eye-catching expressions on it. Bright colours, simple title or phrase in a big, readable font. Yes, it feels "ughhh" sometimes, but it does work.
Copy Tapas' marketing:
I actually tried making an ad on Instagram that copied Tapas' comic ad format (a format used a lot by other webcomic sites) and it worked pretty well! So if you're ever stuck for ideas... have a look at how the platform advertise their own stuff! 
Sometimes you have to sell the audience what they think they want.
In the 2010s there was a weird phenomenon with Disney trailers, particularly Tangled and Frozen. They're both quite classic Disney movies with songs and emotional moments, but when you watch the trailer for Tangled, it's a scene that wasn't in the movie that's a slapstick comedy set to energetic pop rock like something out of Shrek, and the trailer for Frozen is this silly scene between the mascot characters.
A common marketing tactic is to make what you're selling look more like something familiar to make people look at it. It's hard to sell a serious story, so making the promo focus on the easy to get into comedic elements, or the cool character designs or similar can get people to try it, and then the serious story will get them hooked.
Keep going!
A lot of people won't try something out until they've seen it two or three times. Promote and market regularly. After a little bit, seeing your promo will spark recognition and make the person think "I feel like I've heard of that..." and they'll hopefully give it a shot.
Don't chase people who aren't interested:
Some people will NEVER be interested in your comic. Don't chase after them. Shrug it off and look for people who are.
