The new app does not allow to connect to an ongoing measurement, unless you stop it.
For example, if you arrived 3 hours early, you would need to cancel the ongoing measurement in order to generate the report. If you cancel a measurement early you will get a notification that the test must have been running for a minimum of 24 hours or else data will be lost. If there are 25 or more hours of data recorded, then no data will be lost, the monitor will reboot and then you can access the dataset.
NOTE: Many jurisdictions require a minimum of 48 hours to yield a valid radon report. Even if your jurisdiction does not require that, Airthings recommends that all short-term radon tests are terminated in 24-hour increments to best account for day-to-night fluctuations in radon levels.