If the V4S Kiosk is not connected to the network, you see the loader after you Signup/Check-In or sign a waiver form. And the Check-In or the Waiver signup details are not saved in Salesforce.

As a workaround hit the back button and go back to the Job/Shift details screen and continue with volunteering. Once the device is connected to a proper network, the V4S Kiosk app gets updated with the Signup,/Check-ins. And the waiver acceptance details are also saved in Salesforce.