Ask HN: How to properly reach early users (or when to quit the niche)

Hey guys, my first question here, hope it's the right place to do so.

I'm building a web app for teachers, and lot's of questions arised.

Just starting the grind to find new users, but it's frustrating to say the least.

I've found that a vast amount of teachers can be found on Instagram, so I decided to start there.

Already ran a small ad, received some visits to my landing page, but no signups (even for free accounts).

Messages on IG also go into the void, it's really hard to tailor a convincing one.

I've no experience with marketing, so I understand that could be something with that (quality of message, landing, conversion, triggers, etc.).

But then I'd like to know if any of you already worked with this niche, because I'm suspicious that it's not a great one to get started, in first place.

How can I isolate the factors to reach a clear vision of what I am doing wrong, as it can be a lot of things and after that, how can I be sure that product/niche aren't a good fit?

Thanks for your time.