If your cellphone is giving you the error "Make sure the APK is signed" on the mobile app, it is because it has not gone through the initial setup process.
To do this, sign out of the InsiteVR app and click "First time user" on the login screen. You will be asked to enter your email. Once this is done, you will receive a confirmation email within 24 hours. You will be able to use the app on this phone once you've received the confirmation email.
This additional step only has to be done once per phone.