DynamicDNS client does not use custom check IP service
From a pfSense install behind NAT, at Services > Dynamic DNS, add a dynamic DNS client, then on the Check IP services tab, add a custom check IP service. Disable the built-in "Default" check IP service and enable the custom IP check service. Go back to the Dynamic DNS Clients tab and click edit, then Save & Force Update.
The System Log (Status > System Logs) will show the IP was checked with the built-in check IP service
/services_dyndns_edit.php: Dynamic DNS he-net (ddns-client.domain.tld): 126.96.36.199 extracted from checkip.dyndns.org