You're right, they shouldn't have a default password. If they were brand new and you connected them to the NVR originally, then they may have picked up the NVR's password or its activation password - have you tried these in SADP?
If your cameras have a physical reset button (if there's a microSD card slot it will probably be beside this), then you could use this to default the cameras to inactive and set a new password on them in SADP.
If not, you'll need to use the SADP tool to do an XML reset on the cameras by following these steps and sending the reset file to your supplier:
(IMPORTANT - We will only perform password resets for customers who bought the device they are resetting from us - use-IP Ltd - If you didn't buy from us you will have to send your reset file to the company you did buy from) Cybersecurity is a big issue for IP manufacturers at the moment and a...
www.use-ip.co.uk
Then try adding the cameras via the 'manual' method explained here:
There are a couple of methods for getting your IP camera connected and recording to your NVR: Plug and Play - This is the most popular and simplest method for smaller setups, as all you need to do is plug your camera straight into the PoE ports in a Hikvision NVR. The NVR will then do the rest...
www.use-ip.co.uk
You'll need to have IP addresses matching your network's subnet range, and also the gateway set correctly for them to add.
If adding them under the Hikvision protocol doesn't work, you may need to add them as ONVIF cameras by
enabling ONVIF on the cameras directly and selecting this from the protocol dropdown when you add the camera.
Also try updating the cameras' firmware when you can access them to
V5.7.21.
