Check the Ethernet setting in Windows. Make sure it is private, not public.
Windows 10: Settings → Network & Internet. Properties if you need to change it.
Remove server IP address from Settings → Network → LAN Networks.
Leave it blank. Your local clients are on the same subnet as your server, 192.168.1.x.
From the description: “If set, all other IP addresses will be considered to be on the external network…” By adding the server IP address, you are potentially telling Plex Media Server that all other addresses, including local clients, are external.
Check your WiFi router/access point for “AP Isolation,” “Set AP Isolated,” or similar terms. This prevents clients on the WiFi network from communicating with each other or devices on the LAN.