Hi,
I've reported your issue as a suggestion to RnD.
Talking about the work-around:
You can change DynamicDNS config through CLI or WinSCP. Just go to /etc/config/ddns and edit this file with command vi or WinSCP text editor
After you've changed it and saved it - execute /etc/init.d/ddns restart in CLI and it will restart your DDNS service.
EB