I have no experience with that app. Therefore I cannot comment on it.
It is unlikely to be a problem with your P3S. That drone has on-board waypoints. I have a P3S and use it all the time for Litchi waypoint missions.
You don’t say what version of firmware you have. But, you do say that you’re a developer. That normally suggests that you are detail oriented because as a developer, you have to be. Why not troubleshoot this yourself? Do a search for P3S firmware release notes. Find out what version introduced waypoint flights and compare that version with what you have. That is what I would do. I don’t remember the history of P3S firmware updates or whether waypoint flights were introduced with an update or if the P3S had them from the start. You should have all this information from within your grasp.
Make sure your firmware is updated to a version that supports waypoints.
Download and install either DJI Go (v3) or Litchi.
Power-up your drone, make sure you have sufficient satellites.
Load and fly a waypoint mission.
When you have successfully flown a waypoint missions, contact me for payment.