Although I have not personally tested on my Unifi U6-Pro, I think you can use the Unifi Network Controller to create a 2nd “VLAN-only Network” with a separate DHCP server to be the GL-AX1800 and to create separate WiFi SSID’s assigned to the 2nd Network.

The GL-AX1800 WAN port would be plugged into a UDM-Pro LAN port. Both a GL-AX1800 LAN port and a UDM-Pro LAN port would be plugged into the US-8-150W. The UDM-Pro and GL-AX1800 would each have its own DHCP server with separate, non-overlapping scopes within the same subnet.

Depending on which SSID it is connected to through the U6-Pro AP’s, a client device would get a different IP address and gateway address to either the UDM-Pro or the GL-AX1800.

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