[iwalton3]

Plex uses a combination of wildcard certificates and a custom DNS resolver to offer HTTPS on local networks, but it does require a working internet connection to work. [1]

You can also get a certificate through the Let's Encrypt DNS challenge without having to expose a server to the Internet, but you'll still need ownership of a domain name and either an internet connection or a local DNS server to support HTTPS using that certificate.

There is always the option of creating a local certificate authority for your devices, but this is kind of a pain. There are some new applications that aim to make this easier [2], but there is no easy way around having to install the root certificate on each device.

[1] https://blog.filippo.io/how-plex-is-doing-https-for-all-its-... [2] https://github.com/smallstep/certificates