Maybe somebody found a solution to my problem. I am simply not able to update a dynamic IP through OPNsense (however, it work for my Google Domains, so I expect the issue to be related to the configuration "Custom" and "Custom (v6)" under the Dynamic DNS Service on OPNsense.

Here is my update URL (as stated on this page: https://www.dynu.com/DynamicDNS/IPUpdateClient/PFSense-IPv6):
https://api.dynu.com/nic/update?hostname=my-site.dynu.net&password=MY-IP-UPDATE-PASSWORD

In the logs, I am always getting the following if I insert the Result Match parameter "good|nochg|good %IP%":
/usr/local/etc/rc.dyndns: Dynamic DNS: (Error) Result did not match.

If I do not insert the Result Match parameters, I get the following (which sound good! but unfortunately does not change the IP address if I check on the dynu.com website):
/services_dyndns_edit.php: Dynamic DNS: (Success) IP Address Updated Successfully!

Full log:
2021-05-08T12:12:33 config[29964] /services_dyndns_edit.php: Dynamic DNS: (Success) IP Address Updated Successfully!
2021-05-08T12:12:33 config[29964] /services_dyndns_edit.php: Dynamic DNS: updating cache file /var/cache/dyndns_wan__6.cache: xxx.xxx.xxx.xxx
2021-05-08T12:12:33 config[29964] /services_dyndns_edit.php: Dynamic DNS (): xxx.xxx.xxx.xxx extracted
2021-05-08T12:12:33 config[29964] /services_dyndns_edit.php: Dynamic DNS (): Current Service: custom
2021-05-08T12:12:33 config[29964] /services_dyndns_edit.php: Dynamic DNS (): _checkStatus() starting.
2021-05-08T12:12:32 config[29964] /services_dyndns_edit.php: Dynamic DNS ( via Custom): _update() starting.
2021-05-08T12:12:32 config[29964] /services_dyndns_edit.php: Dynamic DNS (): running dyndns_failover_interface for wan. found igb0
2021-05-08T12:12:32 config[29964] /services_dyndns_edit.php: Dynamic DNS (): xxx.xxx.xxx.xxx extracted
2021-05-08T12:12:32 config[29964] /services_dyndns_edit.php: Dynamic DNS: updatedns() starting


It seems that the configuration is extremely simple, and almost everybody is doing the same on OPNsense, however, it is not working in my case and I have no more idea what to try. I have also tried to send the update through the LAN instead of the WAN (still no result).

Does somebody have a working configuration for Custom and also for Custom (v6)? My IPv4 and IPv6 addresses on WAN side are detected correctly.

Your help would be greatly appreciated.
Best regards
Vincent

I just figured out where the issue was coming from.

I was using a specific "IP Update Password" consisting in all kind of characters: 0-9, a-z, A-Z and special characters. The issue was created by the special characters: they are wrongly interpreted if trying to use the update URL in a browser, and they are also disturbing the OPNsense Dynamic DNS updater. Once removing all the special characters, the IP update just worked as expected!

Not IPv4 and IPv6 is updating smoothly, with the Result Match parameters set as suggested by the Dynu tutorial page: good|nochg|good %IP%

Be carefull with special characters when used in passwords for UP Update: they can work on one device (this was my case on my Fritzbox) but they will bring you big troubles on other devices (like in OPNsense).