Temporarily change the PC IP address to be in the same range, for example 192.168.1.200
Access the camera web GUI on 192.168.1.1 (or whatever it really is) and log in.
You may have to use IE11 and download/install a plugin if it insists before allowing the login.
Access the network menu and change the IP address to one of your choice that's unused and valid for your LAN.
Change the PC IP address back to what it was.
Access the camera web GUI on its new address.