Conclusion is MAXIMUS redirected my focus not on port forwarding issue, but a loopback issue. Basically I couldn’t hit my server from within the network, but I could from externally.
I then found:
Which made me adjust to Pure NAT in the System->Advanced->Firewall & NAT tab. Also had to add a checkbox to “Enable automatic outbound NAT for Reflection”. Now my DuckDNS HA link works again. Thanks very much.