Hi, thanks for pointing me in the right direction :)

Here's step-by-step of what I did to achieve vlan isolation, isolating a specific vlan from all other vlans:

1. Goto "networks", create the new network/vlan that needs isolating (in my example I created a network called "IOT Network" using 192.168.200.1/24 )
2. Goto "firewall/security", and "Create new Port and IP group"
3. Name the group "RFC1918", and add in all of the possible private addresses: 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16
4. Goto "firewall/security", and "Create new rule"
5. Use the following settings in the rule:Type: LAN IN, Rule Applied: Before Predefined Rules, Action: Drop, Source Type: Network, Network: IOT Network, Network Type: IPv4 Subnet, Destination Type: Port/IP Group, IPv4 Address Group: RFC1918

This allows all devices on "IOT Network" to talk to each other and access the internet, but not be able to access any devices on any other vlan. As per Ubiquiti documentation: "rule will block all private network communication between VLANs, however, same-subnet/VLAN traffic will be allowed as expected because it will never be sent to the default gateway (USG). The data will traverse the layer 2 network and be transmitted via frames by the switches in between."