How to Convert Free Users to Subscribers?

I have created iOS app for stock research called Company 360. This is what the app does: Processes publicly available data on selected company, evaluates indicators and executes formulas to determine if stock is undervalued or overvalued and provides detailed explanations of formulas used. This app is useful for novice investors with access to only bare-bones trading and no research. With popularity of trading and platforms like Robinhood, the app seems to be in the right spot. There are daily downloads and about 2k active users. However, only a 100 signed up for subscription. The only trend that I’m seeing is gradual replacement of monthly subscribers with annual ones. I don’t know how to convert more users to subscribers. Currently I decided to add new subscription features with every update. This should give users more value for the subscription fee. Aside from that, I don’t have a strategy. My background is FinTech - software development and architecture for banking and Investment research applications. Marketing is not my strong point.