In Google Assistant when I try to link, I get “Couldn’t update the setting” errors after I successfully login to link my HA account.
In the Google Actions Console, the Fulfillment URL I have set is myserver:443/api/google_assistant, per the instructions. When I try to hit that page, HA shows a 404 error.
I was having the same issue and finally solved it.
I am using duck DNS but that shouldn’t pose too much of an issue, you should be able to leave out the :443 part as that is the standard https port. Also make sure you are actually forwarding port 443 to port 8123 for Hawaii. That fixed it for me