Plex was working great until I reinstalled Windows 10 - now videos all look blurry

It is. It means
(a) your PMS considers this to be a remote stream and
(b) the client/server are not able to establish a direct connection – those are limited to 1 Mbps (2 Mbps for Plex Pass members).

If it’s not actually a remote stream, you’ll need to review your network setup for Plex to recognize it’s supposed to be a local stream.

Indirect connections are usually linked to a bad remote access setup. There’s the general troubleshooting article with some hints and a guide on how to approach the actual troubleshooting from the tips & tricks section.
https://support.plex.tv/articles/200931138-troubleshooting-remote-access/