How to allow a device on "Guest Wi-Fi" network on different subnet to a specific device on another subnet?

I am using a Asus GT-BE98 router. I have a Plex server running on a Linux box on a subnet of 192.168.100.x and have moved my TV to a dedicated Guest WiFi network with a DHCP of 10.10.30.x . This Wi-Fi has been set to not have access to the local network (which I think is the issue) and therefore can't see the other subnet. In my ignorance I thought I could create a virtual eth0 on my Linux box in the 10.10.30.x subnet, but obviously this is not enough. Sure I can move the TV back into the 192.x subnet and all will be fine, but I'd like the isolation of it being on it's own subnet for many reas

Question by Astroyka on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.