I have ha running behind a google-wifi-router on ip adres http://192.168.xx.xx:8123/
the google-wifi-router has wan-ip http://109.129.xxx.xx
given by the internet router
on the google-wifi-router a port forwarding configured (on port 8123)
so accessing on the local network http://109.129.xxx.xx:8123 is indeed home assistant starting up., proving that the portforwarding is working 
the setup is if i understand wel een double nat
on duckdns.org a domain famx.duckdns.org is configured (and ping to this domain is working find)
the home assistant duckdns add-on is configured with
domains:
- famx.duckdns.org
token: 57xxxxxxxxxxxxxxxxxxxxxxxxxxx
aliases:
lets_encrypt:
accept_terms: false
algo: secp384r1
certfile: fullchain.pem
keyfile: privkey.pem
seconds: 300
nevertheless http://amx.duckdns.org:8123 is not working
What did i forget in the setup ?
txs for the support