DuckDNS is a bit odd.
If your FQDN is ‘office-hq.duckdns.org’, then the IPFire Dynamic DNS Hostname must be set to ‘office-hq’, i.e., your subdomain. DuckDNS can do this because they offer only one domain: ‘duckdns.org’.
The good news is that when you set the IPFire Hostname to ‘office-hq’, IPFire will update your DuckDNS dynamic DNS.
The bad news is that when you make that change, IPFire will update your DuckDNS dynamic DNS EVERY FIVE MINUTES. That’s because to determine if your IP has changed, IPFire does a DNS lookup on, in your case, ‘office-hq’, and compares the result to your IP. That DNS lookup of course fails. In other words, IPFire’s Dynamic DNS can’t accommodate DuckDNS’s oddity.
I just discovered this.
I’m running IPFire 2.25 Update 144.