DuckDNS Hass.io with internet modem and router. not working

Good morning all,

I know that the subject has been treated thousands of times but I cannot find my solution…

Here, I use home assistant on raspberry pi 4 with hass.io.
I would like to access my server from the outside thanks to the DuckDNS add-on but nothing works.

My installation looks like this:

INTERNET MODEM <=ETHERNET=> XIAOMI ROUTER 3 <=ETHERNET=> RASPBERRY PI 4

This is my last test :

DuckDNS add-on - configuration : 

accept_terms = true
token = XXXX-XX-XXXX..
domains = xxx.duckdns.org
/config/configuration.yaml : 

http: 
   base_url: https://xxx.duckdns.org:8123
   ssl_certificate: /ssl/fullchain.pem
   ssl_key: /ssl/privkey.pem
Internet modem - Port forwarding :

external port : 443
internal port : 8123
IP : 192.168.1.1 (Xiaomi router) 
Xiaomi Router - Port forwarding :

external port : 8123
internal port : 8123
IP : 192.168.1.31 (Raspberry pi - Hassio server) 

I have tried many things.
In configuration.yaml : I put the port, I removed the port, I put https: //, I removed it…

I tested different port forwarding but I’m not sure.

please can you help me?
thank you!

I have the same problem. Do you find a solution?