Just to clarify, do you have your duckdns address pointed at your own external IP address? And there’s no split DNS in play? The duckdns address resolves to the same IP both on and off the LAN?
We had another user very recently with DD-WRT and DNS rebind protection in action preventing them from accessing their site, but it looked like a different result.
Specifically, they got a webpage, served with a DD-WRT certificate, that indicated it rejected a request from a LAN IP to its public interface - check it out: Caddy-docker-proxy: Connection Refused: "type":"urn:ietf:params:acme:error:connection" - #11 by the-bort-the
Your ERR_CONNECTION_RESET looks like a different symptom.