The router supports 4 SSIDs per band. To support 5 SSIDs, at least one SSID is only available in one band.
To use VLANs on your APs, just mark the LAN port the AP connects to “Tagged” (on the Network Settings page). Then assign VLAN IDs to the SSIDs broadcasted by the APs.
The pcWRT router comes with 5 per-configured VLANs, the VLAN ids are:
LAN: 1
Guest: 3
X1: 4
X2: 5
X3: 6