Just turn the feature on under the menu called "Dynamic DNS settings" and it will pop up.
This is from page 66 in the manual:
To activate the Remoterig dynamic dns client select “ddns.remoterig.com” from the drop
down on the Dynamic DNS settings page. Also select a DNS check interval, if your IP address
change often you can select a low value, if it goes month between the changes you can select
a higher value. After that click on “Submit” and “Apply changes”, the RRC will restart.
Btw... It's only available in the Radio-RRC!
73 de Adde, SM0SHG