Those warnings refer to remote access issues.
The primary question will be if you experience those warnings with actual remote connections or even inside your own home network.
If you see this inside your home network, you’ve probably some kind of network segregation in place (client and server being located on different subnets) or there’s some kind of security app/tool that might be preventing the two from talking directly to each other.
The most common use case for that are multiple network components that each create their own subnet (e.g. a wireless access point connected to your router). Unless you specifically want that segmentation, the secondary devices should usually be configured in „bridge mode“ (meaning: they should extend the main router’s network, not create a new one).
If this is an actual remote connection… there’s something wrong with the remote access configuration of the server you’re trying to access. Did you already have a look at the related troubleshooting support article?
https://support.plex.tv/articles/200931138-troubleshooting-remote-access/
There’s also a guide with some more details on how to troubleshoot a remote access issue — even if it might not fix things for you, the findings from the different steps should help pin down what’s causing the issue.
