The inbound and outbound firewalls are set to “allow all” by default. You can change them to “block all” and create custom rules in the Network>Firewall section.
If you have a dynamic WAN IP address, I normally recommend to configure your router in Bridged mode, if possible. Here is a link how to do it with your router. http://www.ehow.com/how_6902901_use-linksys-wrt54g-ethernet-bridge.html
When your router is in bridged mode all your WAN settings can easily be configured on the Peplink. For an ADSL connection change the WAN connection method to PPPoE, and enter your username and password in the section provided. If you configure your ADSL authentication on your Linksys router, you’ll need to configure the router with a static LAN IP, and then configure the WAN on your Peplink also with a Static IP (and gateway to your Linksys router). Remember to create port forwarding from your Linksys to your Peplink WAN IP if you want to make use of it, otherwise you will not be able to access your Peplink from the Internet. You can configure port forwarding to devices on your LAN on the Peplink at Network>Inbound access> Port forwarding.
You’ll need to enable DHCP on your Peplink LAN config, if you do not have any other DHCP servers on your network.
Hope this helps to get you started with your Peplink.