I would like to use my Actiontec C1000A strictly as a modem
Suggest that you first check with your Internet Service Provider to see.....
1) Whether this is even possible
2) Whether they will help you with this configuration
3) Whether they will support this type of setup in the event of problems
As you might imagine on an Apple support forum, we cannot offer much help for an Actiontec product.
IF....it is possible to configure the Actiontec as a simple modem, then AirPort Utility, the setup application for the AirPort Exteme will detect the modem and automatically guide you through the correct PPPoE settings to use.
You will need to have your ISP user name and password handy, as these items will be required during the setup.
Things would be much simpler if it were possible for your ISP to provide you with a simple modem......not a modem/router like you have now.