Connectivity issue with Synapse (federation)

Hello traefik fellows, i'm struggling since many hours(days) on this issue and i hope someone would help me.

I'm using Traefikv2.2 with Docker on a Ubuntu-vm and i have successfully installed Synapse and Element but unfortunately i can't enable the Federation option in Synapse because of a reverse proxy issue or maybe an incompatibility with my domain-name from Duckdns

My Synapse domain is available and secure on the following url:
https://synapse.test.duckdns.org/_matrix/static/
https://test.duckdns.org/_matrix/static/

I can display the Federation message externally on:
https://synapse.test.duckdns.org/_matrix/key/v2/server
https://test.duckdns.org/_matrix/key/v2/server

and internally with the port 8008 open on router:
http://192.168.1.2:8008/_matrix/key/v2/server

The problem comes when Synapse needs to reach the same adresse with the external IP on port 8448. When testing my server on the federationtester service i receive this error:

Get "https://11.11.11.111:8448/_matrix/key/v2/server": dial tcp 11.11.11.111:8448: connect: connection refused

Here are my config files:

traefik.toml

[providers]
  [providers.docker]
    endpoint = "unix:///var/run/docker.sock"
    watch = true
    exposedByDefault = false

  [providers.file]
    directory = "/conf"
    filename = "dynamic.toml"
    watch = true

[entryPoints.web]
  address = ":80"
  [entryPoints.web.http.redirections.entryPoint]
    to = "websecure"
    scheme = "https"

[entryPoints.websecure]
  address = ":443"

[certificatesResolvers.leresolver.acme]
  email = "glacier@gmail.com"
  storage = "acme.json"
  [certificatesResolvers.leresolver.acme.httpChallenge]
    entryPoint = "web"

rules.tom

[http]
  [http.middlewares]
    [http.middlewares.gzip-compress]
      compress = {}
    [http.middlewares.redirect-to-https]
      [http.middlewares.redirect-to-https.redirectScheme]
        scheme = "https"
        permanent = true

[http.routers]  
  [http.routers.synapse]
    rule = "Host(`synapse.test.duckdns.org`) || (Host(`test.duckdns.org`) && PathPrefix(`/_matrix/`))"
    entrypoints = ["websecure"] 
    service = "synapse"
    [http.routers.synapse.tls]
      certResolver = "myresolver"
[http.services]
  [http.services.synapse.loadBalancer]
    [[http.services.synapse.loadBalancer.servers]]
      url = "http://192.168.1.2:8008" 

[tls]
  [tls.options]
    [tls.options.nowadays-tls]
      minVersion = "VersionTLS12"
      cipherSuites = [
        "TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384",
        "TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305",
        "TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305",
        "TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256",
        "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256",
      ]     

docker-compose.yaml

version: "3.3"
services:

  traefik:
    image: "traefik:v2.2"
    container_name: "traefik"
    command:
      - "--api"
      - "--providers.file=true"
      - "--providers.file.filename=/conf/rules.toml"
      - "--api.insecure=true"
      - "--providers.docker=true"
      - "--providers.docker.exposedbydefault=false"
      - "--entrypoints.web.address=:80"      
      - "--entrypoints.websecure.address=:443"
      - "--certificatesresolvers.myresolver.acme.tlschallenge=true"
      - "--certificatesresolvers.myresolver.acme.email=glacier@gmail.com"
      - "--certificatesresolvers.myresolver.acme.storage=/letsencrypt/acme.json"
    ports:  
      - "443:443"
      - "8085:8080"
    volumes:
      - "./letsencrypt:/letsencrypt"
      - "/var/run/docker.sock:/var/run/docker.sock:ro"
      - "/home/linux1/docker/traefik/rules.toml:/conf/rules.toml"
    restart: unless-stopped    

  whoami:
    image: "containous/whoami"
    container_name: "simple-service"
    labels:
      - "traefik.enable=true"
      - "traefik.http.routers.whoami.rule=Host(`test.duckdns.org`)"
      - "traefik.http.routers.whoami.entrypoints=websecure"
      - "traefik.http.routers.whoami.tls.certresolver=myresolver"  
    restart: unless-stopped    

  synapse:
    image: matrixdotorg/synapse:latest
    container_name: synapse    
    restart: "unless-stopped"
    networks:
      - default
    ports:
    - 8448:8448 
    - 8008:8008               
    labels:
     - "traefik.enable=true"    
    volumes:
     - /home/linux1/docker/synapse:/data

networks:
  default:
    external:
      name: compose_default

I really wish someone could help me to find a solution, this issue is making my head :exploding_head:

Thank you for your help.