normal stand-alone MT AP
That is when the wireless interface is in mode “AP” , it has an SSID and a security setting on that interface, also the frequency is normally set
That wireless interface is then, in the normal or usual case connected as port to a bridge.
(Now that wireless interface is “slave” to the bridge. Therefore the IP address and DHCP server will be set up on that bridge (not on the slave interfaces))
Other interfaces, like ethernet interfaces could also be added as port to that bridge, and will use the same IP address , DHCP server and pool
In the second method (where the wireless interface is a normal AP setup) , then a second virtual (wireless) interface is added.
This virtual wireless interface will replace the WAN ethernet interface which connects to Internet
That new virtual interface uses the first wireless interface (with the mode AP) as the master wireless interface, and inherits most wireless parameters from there.
That virtual interface is in station mode, and is used to connect to the Internet providing wifi network (you call it ‘guest’ network)
Bridging over wireless connections is always difficult, so let’s take here the use of a routed setup.
For this, that virtual interface is NOT a port on the bridge, but is given a DHCP client, so it gets IP address and route information from the DHCP server of the ‘guest’ network.
One problem here is to make the virtual interface use the same wifi channel as the ‘guest’ network.
This is done by setting the first wireless interface (the one with the AP mode) to exactly the same frequency and channel width as the ‘guest’ network
On the second virtual wireless interface the SSID and security profile that matches the ‘guest’ interface is set up. This should allow for that wireless connection.
Routed solution here is the normal “AP home” gateway setup, but with the WAN ethernet interface replaced by the virtual wireless in “station” mode.
Where the bridge is a member in the LAN interface list
Where the virtual interface is a member in the WAN interface list
These interface lists are used in the default firewall setup.
When using the default config with the default firewall rules, this should properly set security, NAT (masquerade rule) for the WAN interface, and allow access for all things on the bridge