Installing the APK will allow you to send the GPS position as SMS or to your own HTTP remote service. You will need to allow "Install unknown apps". The reason why the Google Play version does not allow sending SMS is the need for the SEND_SMS permission which will not approve for this purpose.

Unfortunately, Google Play does not allow this app to send SMS (due to permission SEND_SMS is needed). However it allows pushing your location to your own HTTP/API remote service.

If you want that the app will run continuously and not shut down automaticity please enable unrestricted battery access
Settings => Apps => Remote GPS/SMS Tracker => Battery => Unrestricted