If the pRMT does not have network connectivity it will disconnect itself (this is considered as a normal behaviour for the app).
App will reconnect automatically on re-authentication.
Call participant to see if app asks for a QR code > yes > send the QR code
If the pRMT is not asking for a QR code, simply opening the app again should start an automatic re-authentication. If so, ask participant to check whether battery optimisation is turned OFF for pRMT (Settings > Battery > 3 vertical dots > Battery optimisation > Check RADAR CNS Passive is in the list of Not Optimised).
If the QR code screen is not visible or you are not able to continue from there, please contact technical administration (WP7) to help you with the problem.
Closing down the app
Inform you participant that passive app is still running in the background regardless if she/he closes the pRMT app (e.g., by pressing the square button and swiping it off the screen). For participants, it is good to say that it is okay to close the app from the screen and that will not stop the data gathering.