I actually have multiple WAN’s but in failover mode through mwan3.
Not sure what’s your question, really.
DuckDNS will only allow 1 IP (afaik). Assuming you use a site like http://checkip.dyndns.com to get your public address, it will be the one of the WAN having the greater priority at that time.
Your LB router will make the forwarding, I guess
I was approaching this backward; the WAN I link will always point to the load balancer which should provide a straight path to HA regardless. Needed to sleep on that one I guess. Thanks for responding.