I’d make sure you’re forwarding http (port 80) to that same internal host too. I’m assuming thats your jellyfin server. Your browser might not be appending https to your domain and connecting to your router port 80. You can test this by going to both https://<your domain> and http://<your domain>.
On your local network, does going to https://192.168.1.4 present you with what you actually want?
I’m using mailgun and have had zero issues with it. Hard to beat since it’s free.
If vising jellyfin.mydomain.com presents you with your routers config that means you don’t have port forwarding working correctly for ports 443. You should google your router’s name + “port forwarding”.
If you’re a beginner or just for most use cases, using cloudflare with proxied dns records along with Nginx Proxy Manager will provide a good amount of coverage for your homelab.
So the reason you’d want a reverse proxy is because it handles security and would do a much better job of it than an exposed jellyfin port.
Public FQDN -> your home IP -> your router allows 443/whatever to your reverse proxy -> it handles SSL and being hit by the internet (look into nginx security and even fail2ban) -> proxy serves up whatever insecure site/app you’d like.
You know I think you’re right. I might be grandfathered into an old plan. I’ve been using mailgun for over 3 years