See Client says it won’t play from “remote server”, although the server is local .
On the PC running Plex Media Server, in Windows Settings → Network & Internet → Properties, make sure the network is Private, not Public.
If possible, configure the PC to use a public DNS server such as 1.1.1.1 (Cloudflare), 8.8.8.8 (Google), or 9.9.9.9 (Quad9).
