I don't have NVR and my cams are fw 5.1 but this works with them
Code:
curl -T telnet_on.xml http://user:psw@IP:port/ISAPI/System/Network/telnetd
telnet_on.xml looks like this:
<?xml version="1.0" encoding="UTF-8"?>
<Telnetd version="2.0" xmlns="http://www.isapi.org/ver20/XMLSchema">
<enabled>true</enabled>
</Telnetd>