ExtremeWireless (WiNG)

Update:
Kudos to Angelo Cargnel who helped me figure it out in another thread.    

Here are the steps I followed to make this setup (aka Router on a stick) successful: 
1 - Create VLAN2 on AP510 and assign a static IP address to it:  https://extremeportal.force.com/ExtrArticleDetail?an=000082491  

2 - Config the 2nd DHCP pool IP address to be used by VLAN2 devices
https://extremeportal.force.com/ExtrArticleDetail?an=000063527&q=dhcp%20pool%20wing

3 - Config GE1 port as trunk port at AP510
https://extremeportal.force.com/ExtrArticleDetail?an=000080209&q=wing%20trunk%20port

4 - The switch connected to AP510: Ideally you want to place a managed switch here that can do VLAN tagging at physically port level. The port that connects to AP510 GE1 port needs to be setup as trunk port, too.
If, however, you use an unmanaged switch as I do, you have to ensure the switch at minimum can pass through VLAN tagged traffic to AP510 instead of dropping it. The way I tested it is to bypass the unmanaged switch and connect Device3 directly to AP510's GE1 port. Then I enabled VLAN ID2 on Device3's NIC interface. Device3 can get an IP from the 2nd DHCP pool, and can connect to Internet (meaning that AP510's setup is successful and complete). Then I would add the unmanaged switch between devices and AP510. It turned out that without VLAN tagging on the switch, your devices must have VLAN tag ability to connect to the correct VLAN on AP510. Untagged devices will get an IP from the native VLAN (in my setup it's VLAN1) IP pool.