[uniforesecurity]

You are correct, if you don't use the DDNS service, that means you need to re-type the new dynamic IP address for visiting.

It pop-ups login window, that means you can access the DVR, login failure means you input wrong account or password. Basically, the online login accounts are different from administrator account for DVR. You should check your DVR and find the correct account information (possible add users), hence you have the permission to access the DVR.