I read the guide at the page 20, did you tried it on your own AP ?
My AP was already in Router Mode and when i go through local interface as specified in the guide, we can only see the config in read only.
If we want to modify settings and set a custom IP for the DNS, we MUST switch to local management of the AP. To do so, we must reinitialize it.
It confirm we cannot set IP for DNS if we want to manage the AP cloud base and with the app.
It would be so simple to edit the DNS IP field as we can do for the subnet ...