Have you granted the plex web app the permission to access the local network?
You may need to do this for both app.plex.tv and the IP address of your plex server: Important note about the Plex Web app - Local Network Access
Try setting your DNS server to one of the proven-working public ones by Cloudflare or Google (1.1.1.1 and 8.8.8.8 respectively)
Disable 3rd-party anti virus software.
