I had the same problem which had me stumped for longer than I would like to admit. I could login to the DVR over the lan, but not over the internet. The problem was with the way I set up port forwarding on the router. Port 80 was sent to port 80 and the range of ports from 9091 to 9112 were all sent to 9091. The port that is used for logins is port 9092, so you get as far as that and then it fails. You must send port 9091 to 9091 and then send 9092 to 9092 and 9093 to 9093 one at a time and so on all the way through to 9112. Remember to send 80 to 80, I would not use 8080, set your dvr software to use the standard port 80.
Good Luck
Frank