The url that is used for EPG setup / refresh is this https://epg.provider.plex.tv with various end points
An example here Can't add setup Plex DVR and download EPG - I see two errors: Unable to load channel mappings and Unable to fetch guide settings - #4 by sa2000 showed that out of 32 requests to the service, only 6 completed successfully with the rest timing out
An EPG refresh may involve a few thousand requests to the service - over a 20 minute or so period.
Failures that have been seen are early on so was not what could be rate limiting
Only advice is to report this to your provider. The server logs would show what the http requests were that failed
