I have PMS and my TV on separate subnets. Make sure that both subnets are specified under the LAN Networks in the network settings on your PMS.
For example if your server is in the subnet 192.168.1.0/24 and your TV is in 192.168.10.0/24 then you would enter it as 192.168.1.0/255.255.255.0,192.168.10.0/255.255.255.0.
Also make sure that the firewall in your router is configured to allow traffic from your TV to your Plex server.
