My digital ad sales experience occurred in 2017. My job involved 100s of cold calls via phone & emails.
The first challenge was creating a list of contact names of ad buyers (the decision makers) at the numerous organizations I contacted. The next step was trying to establish a line of communication with these ad buyers.

Remember, these folks are shielded by their inbox & voicemail, so it's easier said than done!

Another option available to you is to offload the responsibility to the major algorithm distributors, namely Facebook and Google. The problem here, though, is that they hoover up much of the generated revenue, and pay you pennies per impression.
But it does get you started, and the tracking technology that the algorithms provide can be useful for targeting new users of your app.

Bottom line, my experience tells me that cold calling ad buyers at the companies and organizations who place ads is a very challenging way to go. It may become easier once your app is popular, but attracting them to an ad spend on an unproven app will be tough.

The technology behind digital ad distribution is constantly evolving. My next suggestion is to gain as much knowledge of the future direction of digital advertising, which will greatly assist you in deciding how to proceed.

Best success to you!