What happens when free trial ends
You don't need to do take any action to make sure the app continues to work after the free trial ends. The only significance of the free trial is that there won't be any billing done during the free trial period (14 days) and the billing will start only after that period has elapsed.
If you wish to continue using the app, you don't need to take any action and monthly billing will automatically start after the free trial has elapsed. The app will continue to work without any interruption during this switch.