I’d make sure your 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?
You know I think you’re right. I might be grandfathered into an old plan. I’ve been using mailgun for over 3 years