Thank you for asking this question in the support community. At the time of this post, there is not a command to force a DynDNS update in the CLI of the ADTRAN. The debug dynamic-dns <verbose> command will display output when the DynDNS process is updating. If you enable the dynamic-dns debug, and then remove the dynamic-dns configuration from the interface, followed by re-configuring it, the debug will show the process attempting to update and you can troubleshoot the output to see where it is failing. If you would like to reply with this debug output, I will be happy to review it for you.
It ended up being an issue on DynDNS' side. The account had gotten locked out. Running debug dynamic-dns verbose on a couple of routers showed us the authentication error.
They weren't able to tell us why some were updating and others weren't (they all share a single update account), but the issue's fixed.
I appreciate your help