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Home Assistant is on my home network on a raspberry pi.
At home I access it on the local network, remotely I can access it through SSH tunnel port forwarding. Both are fine but only through http, not https.
There are some integrations (Alexa and Samsung off the top of my head) that require an external, secure connection to Home Assistant to work which I don't think I have at the moment.
SSH tunnels don't need certificates in themselves - at least, I don't have any for the ones I use.
What's the setup you have (where is the HA instance hosted, how are you trying to access it, and where from)?