The DVR is the server. DVRs aren’t really designed to off-load camera feeds and recordings for the sake of security. If you want to build your own server, you’ll need to incorporate analog inputs.

If the issue is viewing the cameras from a mobile device, you can try to get this to work with third party apps. I’ve used an (iOS) app called DMSS to access a DVR similar to what you have.

Also, the benefit of a self contained analog device is, among other things, it doesn’t interfere with your network traffic or rely on your router. Get it a decent battery backup and it can still run when the power’s out. Analog cameras generally look better than IP cameras too.