When assigning an SSID to a VLAN, this has the affect of adding the Virtual LAN inner tag to the header of Ethernet frames once Wi-Fi client traffic reaches the Access Point for forwarding onward over Ethernet.
The 'WAN' side of the Access Point will present an Ethernet sub-interface with the Virtual LAN identifier equivalent to VLAN set duing SSID creation.
802.1Q VLAN SSID is configured within Profiles, SSID Profile:
Create a Profile of type SSID
Assign a Profile Name
Create the SSID
Assign SSID Name
Set related optional SSID parameters
Mode is set as Bridge
Security and Encryption
VLAN is set to 'Use Custom VLAN'
Enter desired value from 2 - 4,095
Once the SSID Profile configured for a unique VLAN is associated to an Access Point, clients will receive DHCP, Routing, DNS from the Virtual LAN of the upstream network provider