Check there isn't a security setting on the DVR that only allows login for certain ip ranges etc.
Also check what protocols you are using. you may need to forward additional ports?
for example you may have forwarded port 80 for Http,
but you may need 443 for https, or 22 for SSH to complete a secure login for example.
if you have command prompt open on your pc when on the local network, login, then quickly type "netstat" in the command prompt window and hit enter, this should show any open connections and what ports they use.
you can also use a program called wireshark but that's a bit advanced / overkill.
what login settings are available on the dvr?