[FWA515] - Public/Private URL for return the WAN External IP

Hello everyone,
I've an FWA515 that use the ETHWAN port to establish a PPPoE connection over internet.

Then I've also an USG Flex 500H firewall, and this is configured in DMZ from the FWA; the FWA return to the flex a fixed IP, and the firewall configure it as external WAN interface.

ONT Fiber → PPPoE → FWA ETHWAN

FWA LAN → Flex (port P1 as external)

The FWA DHCP, have a static fixed IP released on the Flex.
The FWA have this static fixed IP as DMZ.
The Flex use this IP, as external IP.
All the inbound WAN traffic are carrier directly on the Flex because the flex are in DMZ into the FWA.

Now, I've two DDNS directly configured into the Flex.
The service cannot use the WAN IP interface, because the interface have the internal NAT IP (192.168.x.x) not directly the WAN address.
So, I've configured the DDNS service to use a Public URL to manage the WAN IP to assign at the service

Now, it's possible to have an URL directly into the FWA that return the public WAN IP instead use an external URL?

Thank you