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.