SOLVED: Firetick App

[TheDreadPirate]

You may need to use a different reverse proxy. Nginx Proxy Manager is my recommendation. The issue, I think, is that your Firestick probably hasn't received any security updates in a while. Those updates usually include updates to root and intermediate certificate authorities. Let's Encrypt's intermediate authority refreshed in March 2024. If the DSM proxy isn't sending the client the root certificate, only the server and intermediate certificates, then your Firestick won't trust the cert.

In many reverse proxies, you can configure what certs to send to the client. But it does not appear that the DSM proxy can.