I think you might be a bit of confused with the terminology of the rail planner app.
Only one Trip can be connected to your pass. But a trip, can contain multiple journeys over multiple days, during the validity of the pass you connected to it. You have to see the “trip” as your travel diary of your interrail pass.
On the rail planner app, there are different tabs:
Planner, My Trip, My Pass and More
Click “My Trip”, select the trip you connected to your pass to and your journeys for the first day, click the yellow plus sign to add a new journey, and tap the grey ledger to add that journey to your pass. A new travel day will be used and it’ll generate a new qr-code.
No need to connect a new “trip” to your pass for a new travel day, just add new train journeys for the other days on the existing trip you used for the first day.