When you said Jellyfin streaming isn’t working - are you able to actually get to Jellyfin UI and its the stream failing, or you can’t access Jellyfin at all via nginx?
It’s not working because it is against Cloudflare’s ToS unfortunately.
First I would ask, do you really have to make Jellyfin publicly accessible?
If yes, are you able to setup a VPN (i.e. Wireguard) and access Jellyfin through that instead?
If you don’t want the VPN route then isolate the NPM and Jellyfin instance from the rest of your server infrastructure and run the setup you described (open ports directly to the NPM instance). That is how most people that don’t want to do Cloudflare are running public access to self hosted services. But first, ask yourself the questions above.
If you mean you're having trouble getting NPM to work with Jellyfin, here's how I got it working:
Make sure you have “Websockets Support” checked.
Then create a custom location "/", with the following in the advanced config:
## The default `client_max_body_size` is 1M, this might not be enough for some posters, etc.
client_max_body_size 20M;
# Security / XSS Mitigation Headers
# NOTE: X-Frame-Options may cause issues with the webOS app
add_header X-Frame-Options "SAMEORIGIN";
add_header X-XSS-Protection "0"; # Do NOT enable. This is obsolete/dangerous
add_header X-Content-Type-Options "nosniff";
# COOP/COEP. Disable if you use external plugins/images/assets
add_header Cross-Origin-Opener-Policy "same-origin" always;
add_header Cross-Origin-Embedder-Policy "require-corp" always;
add_header Cross-Origin-Resource-Policy "same-origin" always;
# Permissions policy. May cause issues on some clients
add_header Permissions-Policy "accelerometer=(), ambient-light-sensor=(), battery=(), bluetooth=(), camera=(), clipboard-read=(), display-capture=(), document-domain=(), encrypted-media=(), gamepad=(), geolocation=(), gyroscope=(), hid=(), idle-detection=(), interest-cohort=(), keyboard-map=(), local-fonts=(), magnetometer=(), microphone=(), payment=(), publickey-credentials-get=(), serial=(), sync-xhr=(), usb=(), xr-spatial-tracking=()" always;
# Tell browsers to use per-origin process isolation
add_header Origin-Agent-Cluster "?1" always;
# Disable buffering when the nginx proxy gets very resource heavy upon streaming
proxy_buffering off;