@TP-Link You can not be serious? Most of the switches are using VLAN tagging for separating networks. But seems that TP-Links are not meant to be used in such networks. Maybe target audience is just...

How? I don't see any VLAN id. I can only select Isolation form main network? I router mode it can be done but question was AP mode.

How the guest wlan isolation has been implemented in AP mode in deco M9 latest firmware for v1 hardware? Does it have VLAN id and what is it? Why it cannot be set from user interface?