Hello @RADEB 

Is there a switch connected between the ER605 and the UniFi APs?

Did you configure VLAN20 on your SSID from your UniFi APs?

If your network topology is like ER605 ---- (Port 1) Switch (Port 2) ---- AP )))((( SSID vlan20

Please ensure that the Port1 and Port2 of the Switch is configured with VLAN20 trunk ports. And the SSID from the AP has VLAN enabled and configured with VLAN20. Otherwise, the clients connected to the SSID won't get the IP address from VLAN20.

Hello @RADEB 

RADEB wrote

I want to be able to connect a particular device to anywhere on the network and the device will be assigned a fixed IP address on VLAN20 (192.168.20.XXX). Apart from the switch mentioned above which is only for Wi-Fi, the rest of the network is connected directly to the router.

All devices work in the VLAN1 network by default. If you want a device to be assigned with an IP address on VLAN20, please ensure that the device belongs to the VLAN20 network. For your case, you may need to specify the VLAN config on the ports. However, currently it doesn't support to specify the Port PVID on the ER605 router when it's managed by Omada Controller. 

If you want all devices to work in VLAN20 instead, that is, to replace the VLAN1 with the VLAN20, you may forget the ER605 from your Controller, and configure the Network in Standalone web UI. Go to Network > VLAN > Ports to configure the PVID to 20 for a spare LAN port, then modify the default VLAN1 network to VLAN20 (192.168.20.xxx), and connect to the spare LAN port to access the web UI again, finally configure the PVID of other LAN ports to 20. After that all LAN devices will work in VLAN20.