Hi Darren,
Are you trying to set up a regular CalDAV account or a CalDAV subscription on the iPAD?
The regular CalDAV will let you edit, but not the subscription. This is in Settings > Mail, Contacts, Calendars > Add Account > Other > and then you can choose either
Add CalDAV Account
or
Add Subscribed Calendar
.
Not sure about the IPs, but you can't access an external IP from inside the network, so if you're behind the firewall, you need to use the 192, and outside of it, you'll need to access the public one.
If the device is moving in and out of the firewall than a domain name or a service like DynDNS can help with this.
Let me know if that helps,
-Jason