From the short amount of research on your DVR that I have done, I have determined they don’t support an app. They support a web browser GUI which to make accessible from outside your network you would need to open ports on your firewall.
Your best bet would be to replace the system with an NVR & new cameras (Network Video Recorder) which replaces the BNC connections to the cameras with ethernet. Or you could purchase a new DVR with the ability to work with a smart app compatible with Apple IOS devices. This Lorex unit goes for $199.99 and is compatible with your existing cameras.