I lost internet access the other day and couldn’t access plex server on client devices.
A bit of searching quickly led to https://support.plex.tv/hc/en-us/articles/200430283-Network and that I had to manually add IP addresses.
As I have a few devices that use Plex and not all are fixed I tried using CIDR notation and it worked.
If you add “192.168.1.0/24” here it will allow any devices within the 192.168.1.0 to 192.168.1.255 range (which is nearly everyone).
This is a lot easier than manually adding each IP and having to fix all IPs.