I looked at your webcomic..
Now, given that my own webcomic doesn't really have that much subs too, I'm not gonna be the best guy to give you advice on how to gain more subs. 
But from my experience, A comic with a thumbnail that stands out attracts more readers. And with more people checking out your work, you raise the chance of more people ''subbing" to your comic.. So maybe improve your main thumbnail.. With regards to the art of your comic, it's actually pretty good.. Better than a lot of comics i've seen in the discovery section of LINE. at least in my opinion.. Maybe color the earlier chapters.
That's all I can give for now..
I subbed to your webtoon.. I'm curious to see how you'll improve.. 