I have got a domain (www.example.com) and using a subdomain (ha.example.com) pointing to a CNAME record of my Synology DDNS (ha.abc.myDS.me). Then in Synology reverse proxy I have (ha.example.com) pointing to (HA IP address:8123).
A Let’s Encrypt certificate for www.example.com is install in my Synology DS.
I can access the HA login page. But when I type my password, it say connecting and then revert to the login page again. I can confirm it works when I type in the IP of my HA.