To update DynDNS upon IP change IONOS has an API that you can use for that. https://developer.hosting.ionos.de/docs/dns
When you run through their workflow you are prompted with an update URL like this. Calling this will update the A Record of the domains you provided during the beforementioned workflow.
https://ipv4.api.hosting.ionos.com/dns/v1/dyndns?<TOKEN>
To update the AAAA record (IPv6) as well, you need to add a parameter to this update URL.
add &ipv6=__MYIP__ to retreive a URL like this one.
https://ipv4.api.hosting.ionos.com/dns/v1/dyndns?<TOKEN>&ipv6=__MYIP__
Now set up OPNsense like stated below. One account for IPv4 and another one for IPv6 in the DynDNS plugin.
IONOS DynDNS IPv4 mit OPNsense (Native Backend)
config:
Service: Custom
Protocol: Custom GET
Check IP Method: freedns
Interface to monitor: WAN
Server URL:
https://ipv4.api.hosting.ionos.com/dns/v1/dyndns?q=<TOKEN>
IONOS DynDNS IPv6 mit OPNsense (Native Backend)
config:
Service: Custom
Protocol: Custom GET
Check IP Method: Interface [IPv6]
Interface to monitor: WAN
Server URL:
https://ipv4.api.hosting.ionos.com/dns/v1/dyndns?q=<TOKEN>&ipv6=__MYIP__
After years I finally found this solution thanks to Copilot and wanted to give a little bit of what I got from this forum already back to you.