How can I isolate an ethernet connected device into the guest network?

Is it possible to isolate an ethernet connected LAN device into the guest zone?

I have tried assigning a guest zone IP to the said device, like 192.168.9.149, but when it reconnects, it always gets an IP address from the LAN network, in my case 192.168.8.149.

What should I do? It would be preferable to isolate this device.

Please refer to