Just to clarify, it should work fine if secure connections are working correctly on your local network. Start by making sure that your server is signed in with your Plex account, everything on the server side is automatic and there shouldn’t be anything else you need to do.
Normally the main issue with secure connection on the local network is caused by DNS rebind protection on your router, see the “Why isn’t it working” section here: How to Use Secure Server Connections | Plex Support
If you’re struggling after doing this please provide some server logs and I’ll check if everything is looking okay on that side.
Settings → Troubleshooting → Download Logs (please make sure you have Debug logging enabled).
