1. Connect a router to the device.
2. To connect the VPN to the device, first, you need to set up a VPN on the router. Check here for VPN configuration for different routers.
3. Connect the Apple Airport Extreme with the router by bridging.
4. Go to the Network tab.
5. Turn off the Router mode to turn the device in to switch.
6. Connect the Airport with the second router.
Instead, you can also place a switch between your broadband ISP modem and your two routers. The topology goes like this:
Internet Modem –>
Ethernet Switch –>
Switch Port 1 –>
Airport Extreme–>
Switch Port 2 –>
VPN Router