Thanks Chris, I already saw that post earlier, and I have tried implementing a Guest network with VLANs, DHCP on the AP, NAT, ACLs etc., however the ACLs aren't taking effect. Guest clients can still access the default VLAN/corporate LAN.
In any case, it should be much simpler than this. On a Meraki AP it's much easier. You have an option to enable NAT with DHCP and the firewall rule to block access to local LAN, and it just works.
I'm more curious as to why the 'Only Internet Access' shown in the help file is missing from the AP config page.