MT2500 doesn’t have a SWITCH, it only has two PHYs.
Creating a 802.1q VLAN, VLAN id is 12 in this example.
Creating a interfaces for subnet 192.168.9.x
Creating a firewall rule, guest accept output, reject input and forward, and accept dhcp and dns
Now you can get a guest subnet 192.168.9.x from LAN(eth1.12 vlan id is 12).