I'm a great believer in the KISS principle. The following worked flawlessly for me at several locations with both modem/routers and modems:
Start from a factory default condition. Connect the main Airport to AC power and finally to the Internet modem (or modem/router). Open Airport Utility on your Mac or other computer and accept all of the recommendations. After confirming that it works, insert your desired administrative and WiFi passwords.
Then connect the extender to AC power. As before, open Airport Express on your Mac or other computer and accept all of the recommendations. No need to add password info.
By the way, I used Airport Utility version 6.1.
Do not expect the Airport to respond instantly to the configuration commands. BE PATIENT!