Is there any other devices that is working fine with the router? if so, you may check whether the DVR obtains valid IP address then try to ping it on other computer to see whether they can communicate with each other.
I was having the same issue before, then I contacted tp-link technical support and finally I found that it was my IP address issue on the DVR, I set a static IP address on the DVR, but it was not in the same subnet as the router, then I changed it to obtain IP address automatically, now it works fine