I suggest building a landing page and advertising to the landing page. From there, you can have the download buttons for Apple and Google. With the landing page you will have more control over retargeting with the Facebook pixel, Google tag, etc, along with the benefits of SEO if you start content marketing. A great article I recommend for crafting a landing page is My step-by-step guide to landing pages that convert
I also recommend reading the books Building a Storybrand and Marketing Made Simple by Donald Miller.
This Quora article is a little dated, but I think the principles are still in tact. https://www.quora.com/What-are-the-benefits-of-having-a-landing-page-for-your-mobile-app