More than where you advertise and what hashtags you use, I think the key is you need to work out:
- What am I selling?
- How can I get that across in an appealing and very clear way?
This is where the design and tone of voice of the ads and posts you make becomes extremely important.
In your case, I can see why you're struggling. You're making a merman BL story, but you don't even mention it's about a merman until the last part of your blurb, and your cover doesn't clearly show the main couple, just a vague ghost of two faces I can barely make out, and then the font makes it look like a sort of... gothic mystery dark academia type story rather than a romance one.
If you are making a novel that is: Modern setting story about the love between a man and a merman trapped on an island that contains explicit scenes, the cover should have sexy guys in the water or on the beach or something to get that across, and the font should be something more like a romance novel with a modern setting, maybe a handwriting-style font. The blurb should have the word "merman" somewhere in the first two sentences. Look at popular BL with a similar vibe for inspiration.
Fix these things first. Then, using these assets, look to social media and make posts that do things like posting short excerpts and finding hashtags.
